I saw someone recently who had the same problem. Rather than look for smaller car seats, she actually found a website that sells an extra actual seat that can be installed in the cargo hold of many SUV type vehicles. Below is the web site. She said she spent about $500 total, but it was designed that her husband was able to install it himself (it is designed to fit with the pre-existing bolts, etc. of the cargo hold). Below is the web site. I have to say, it looked very nice in the back of her SUV and her older son really liked being back there and having his own space. She said it was totally worth spending that extra money to make it easier, rather than trying to squeeze the three car seats in.
http://www.littlepassengerseats.com/index.html
Also, if you don't want to go that route, I'd suggest looking at second hand car seats. I have a Saturn SL2 (which is the small sedan - not even an SUV), and I've got three car seats in the back seat - if I can fit them in my little Saturn, I'm sure you can find three to fit in the back of your jeep. One is the low booster by Graco - the child is actually buckling in with the car's seat belt (your six year old should be big enough for one of those), one is a five point harness by Graco -my four year old is in this one, but it could take a younger child - she was three when we started using it - it is very plain, the seat is flat - no side arm rests/cup holders, so it fits well next to the baby seat - the infant carrier I used when my son was under a year was by Century, and the car seat I use for him now is by Century - again, this one has sides, but no fancy cup holders, etc. I don't know if they still make those models - I got one from my sister-in-law and another from my sister. I just started collecting hand-me down car seats until I found three that worked. It is a bit of work to reach in-between the car seats to buckle the six year old into her booster, but it works. I just didn't have the money to buy another car when we were expecting our third child and was determined to find three seats that worked together.
Good luck.
I am a working mom of three (6, 4, and 16 months). T