Need Car Seat Recommendation

You're right to keep your son in a harness, he's way too young for a booster. If you're looking to buy 3 new seats, I'd suggest Sunshine Kids Radian, get one w/ the insert for the Newborn (they've been rated as one of the few convertibles that does work well with newborns)...it'd be expensive to buy 3 of those though, but they are very narrow seats, actually designed to use 3 across.
Otherwise, if you don't want to buy 3 new ones, check out www.car-seat.org there are lots of pictures & suggestions of different combinations of three across, and if you post what seats you already have, someone might be able to tell you which seat to get for the 3rd that'll work with what you have. (assuming you need to buy a seat for the kid you watch?) ....

Another thought, if your son is close to outgrowing whatever seat he's in, maybe the other kid could use that, and get Graco Nautilus for your son, it harnesses to 65 lbs and then turns into a Good booster seat (highback then lowback) up to 100 lbs, and usually less than $150. Similiar options, more expensive, are Britax Frontier (Harnesses to 80 lbs, currently recalled but I'm sure it'll be back on the market as soon as they fix the problem, very popular seat) or Safeguard (very pricey but lots of nice features, on sale right now at many online stores for $299)

Hey, I also have a toyota Camry...I don't know how old the little boy you watch is, can he be put in a booster? Maybe try your infant carrier in the middle and the other carseats on the outside. You should be able to fit 3 carseats back there but it will be tight...good luck.

We use the Sunshine Kids Radian seats. You can fit 3 across in our Subaru Outback. They are expensive (about 200 a piece) but the BEST seat we have ever owned. They are 5 point harness, can be used for an infant and go up to either 60 or 85 pounds. Hope this helps.

G.

It can be done. don't give up finding car seats that fit. I had my first 3 children in just under 3 years. For a few months between birthdays, their ages are consecutive (9, 10, 11 next year). We couldn't afford a new vehicle & put 3 car seats in the back of our Mazda 626. We did put our 2.5yo (the oldest) in a booster seat. I can understand you not wanting to do that.

A trick is to not undo it once you get the car seats situated. Do not unbuckle one of those car seats. My brother laughed when he first saw my sedan full of carseats.

some posts mentioned a convertible carseat; you will want to keep in mind that your infant will be ready to face forward before your son or the other boy are ready to get out of carseats maybe before they are ready for boosters. (I don't know if you'll still be watching him at the time.)
--R. J.