M.P.
Does your car have LATCH? I know that my Honda Accord does, but it will only fit 2 car seats utizing the LATCH system (and there is no room for an additional person). If I want 3 people in the back seat, I can only have one car seat in the middle.
You might call your dealer and see if they have any recommendations. Also take the car to Babies R Us. You might be able to take the floor models to the car and strap them in to see if they'll fit before you buy them.
Personally, I'd trade in the car for a 3 row SUV or Mini-Van. Good Luck!