By DR ANDREA BONIOR

Many of our friends, happy in the lives they have with two kids, are nevertheless entertaining the idea of adding a third child to their brood.

Having pushed through the heart of that particular dragon with three kids in a four-and-a-half year span (whether we’ve survived is still being medically investigated), my husband and I are often asked what our take is.

We’ve decided to spell out the differences between two kids and three in a handy-dandy list for your reference; feel free to hang this up by your bed to consult in times of weakness. (And do parents of four, five and six — and more — have it exponentially harder than we do? Of course. But we can’t even fathom those situations without crawling into the fetal position. And that’s not great for typing.)

1. Cleaning. Parents of two kids may often experience the unfortunate sensation of not being able to clean up as fast as the mess is getting made. This “treadmill” effect is unsettling, and leads to feelings of abject helplessness as toys, clothes and books pile up out of control. With an additional child, however, the mess and filth will take on an even more sinister life of their own, with many unknown objects somehow being unearthed. A typical day’s debris will now include not just toys, clothes and books, but toys, clothes, books, used napkins, Band-Aid wrappers, raw meat, medical waste, 77 yards of crumpled Scotch tape and a rusted, crumbling earhorn.

2. Bedtime Stories. When snuggling in to read a book at night, two children can each acquire a comfy spot adjacent to the parent and the book. With three children, one child will always be left out. This brings about a battle not unlike Ultimate Fighting Champions, as the warring factions jockey for space through any physically violent means necessary.

3. Money. Having three children will cost approximately 1.49752 times as much as having two children. Does this make any sense? Not in the least. Are these numbers the result of rigorous mathematical inquiry? Of course not. (We had to sell our calculator to pay for the third kid’s shoes.)

4. The “Parental” Bed. Yeah, we know, we never should have let it get this out of hand, we should have stuck to the “No kids ever in our bed” rule night-in and night-out. Our exhaustion won out, however (see “SLEEP”) and now we have a nightly struggle with the reality that not even a king-sized bed will prevent the onslaught of three interlopers’ elbows, knees and feet poking rhythmically, like frantic Morse Code signals, into our internal organs.

5. Triple Relationships. No one ever seems to discuss this, but it’s a biggie. When you have two children, there is one relationship between them. One. Easy peasy! When you have three children, it doesn’t just double — it sails right over to three. (Count ’em. We’ll wait.) That’s three dyads going at any one time; three intricate emotional terrains; three possible grudges; three possible prank wars; three possible fisticuffs; three possible long, drawn-out retaliations after one person deliberately farted on one other person’s science project.

6. Noise. Parents of two may sometimes complain about having a loud house. Parents of three simply can’t hear them. (Or anyone else, for that matter.)

7. Carparks. When a parent of two saunters across a crowded carpark, both kids can each be granted a hand to hold. When mad-dashing across a crowded car park with three children, however, you shall rely on the “chain” method: “Keep holding your sister’s hand. Grab her hand! Take her hand!!!! STOP. HER. AND! GET!! HER!!!! HAAAAAND!!!!” This basically guarantees a wayward 5-year-old dashing off on her own as you hope against hope there’s no clueless, texting teen driver stepping on the gas a few feet away.

8. Strollers. With two children, you can still somehow squeeze them into a single stroller when desperation strikes and everyone is absolutely refusing to walk that last uphill stretch at the zoo (were the lemurs worth it?). With three children, however, you’ll have no such luck. Oh, you’ll still try to cram them all into the stroller, all right, but the off-kilter sculpture you create with their bodies will cause amused onlookers to stop and stare, thinking it’s either Cirque de Soleil street art or the scene of a terrible accident.

9. School Forms. We imagine that the sheer volume of school forms, art projects, and other papers that come home each day with two children can sometimes feel unmanageable. With three children, however, these stacks of paper will actually mate with each other and multiply while you are sleeping, to the point where old math worksheets will not only become your companions at the dinner table, but your pillows in bed, your washcloths in the kitchen and your loofahs in the shower.

10. Teacher Appreciation Week. Hmm. You certainly didn’t hear it from us, but… MAN. (We love you, teachers!)

11. Feces. Do the math: it multiplies, and it’s everywhere.

12. Vehicles. Two kids, sure, you can keep the Prius. Three kids, and the odds of your succumbing to a minivan will increase considerably. Now you “need” an automatic sliding door the way an eel needs water. Which brings us to:

13. Vehicle Archaeology. Sure, parents of two may have found a couple of disgusting items in the depths of their car cushions, perhaps even fossilized and unidentifiable ones every once in a while. But only parents of three or more will find things so frightening they require a police inquiry.

14. Entertainment and Travel. From ticket packages to hot dog deals at baseball games, American entertainment is set up for foursomes. True story: if you have three or more children, you will never be allowed to tell the truth about your family size while booking a Manhattan hotel room; they simply won’t let you reserve it for five. (This is probably because these rooms are usually smaller than your middle child’s head.)

15. Restaurant Dining. When standing in a crowded restaurant’s waiting area, parents of two shall take comfort in the fact that a simple four-top might actually become available sometime in the next six months. A family of five, however, will need to choose between camping out for the night at the hostess station, or sucking it up and making one kid lay across the table as one of the serving platters.

16. Sickness. Any parent of two children will gain intimate experience with stomach bugs, perhaps even suffering the unfortunate occurrence of having bodily fluids emerging from both ends at the same time. With three children, however, the virulence — and frequency — of said stomach bugs, and every other illness, seems to increase exponentially, as you’ve got more interactions among more kids and their friends. Want to know whether there’s a similarity in the molecular structure of lice, strep, pinkeye and the norovirus? Ask any parent of three or more. They’ll know from their experience of having them all simultaneously — for the twelfth time.

17. Sleep. Wait. What?

18. Babysitting. As parents of two children, you may know several other two-child families where a regular swap of babysitting services, or a last-minute bailout to watch your kids, is an easy, reciprocal deal. With three kids, however, such swaps are all the more rare and difficult — naturally, foisting your three on parents of two brings about a sense of guilt. No problem, you think. Just seek out other parents of three! Except then you realise those parents are already lying down, exhausted, on their living room floors — which brings back the guilt all over again. No babysitting for you!

19. Emergencies. Parents of two might have the unfortunate experience of an occasional visit to the ER, whether for stitches, croup or croup-related stitches. (It happens. Ask a parent of three.) Parents of three, however, will likely visit the ER with such frequency that they’ll soon find themselves begging the staff to implement a rewards punch card that will at least get them a free sandwich every once in a while.

20. Football Analogies. You’ve heard it before: two children and two parents is a man-to-man defence. Three children and two parents, however, is a “What the #$%@ were we thinking?” defence.

21. Relief via Partner. When you have one child and your spouse/partner takes that child out for a while, they are the saviour who is giving you a break. When you have two children, if your spouse takes one of the kids out, at least they’re pulling their weight with 50 percent of the load. When you have three children, if your spouse takes just one of your kids out, go run after him/her. They are slinking away, shirking their duties — and should be punished accordingly.

22. A Reality Check. With two children, there are two pairs of eyes that look out at the world with wonder, and two little mouths smirking in pleasure and surprise when they first learn about marmosets, and two little voices saying “I love you” after you patiently answer their questions about outer space or teach them how to chop an apple. With three, there are three. Yup. That’s pretty much the only difference you need to know. And you know what? We wouldn’t trade any of this for the world. In fact, we will yell it from the rooftops, just as soon as we find a clean pair of pants and stop that ringing in our ears.

Dr. Andrea Bonior  is a licensed clinical psychologist, professor, and writer. You can check out her weekly Washington Post Express column Baggage Check, her book The Friendship Fix, or her blog on Psychology Today. For her television and other media appearances as a licensed clinical psychologist, please check out www.drandreabonior.com.