My friend fit three in the back of her Toyota Corolla. Wow, I didn't think she could do it!
It seems to me that her infant seat fit in fine, but the problem was when she needed a convertible seat. I'm afraid that she kept her son in his infant seat far too long because of it. She finally bought a convertible seat for him and when she had it installed, the doors hit the seats. So she went to the fire station to have it checked out, and they said it is definitely NOT SAFE to have the doors hit the car seats. They gave her a new convertible seat, and said it was the slimmest one on the market. It worked! Three seats fit in her tiny little Corolla.
It just happens to be the least expensive one, and the one we have been using for 4+ years. It's the Cosco Touriva/Regal Ride. I think they've renamed it the Cosco Scenera, and it runs about $50. Now my very large 2 1/2 year old rides in it, although he's about to grow out of it. I think your 2 year old could sit in one and that would give you more room.
I agree that some infant seats are slimmer, too. I don't think I know which brand is slimmer, but I am pretty sure my Evenflo Discovery was a wider one. I think that because I had to buy an Evenflo double stroller because the Graco strollers said my Evenflo car seat was too wide to fit in them. Did that make sense?
You ought to be able to buy one, take it out to the parking lot and install it and see if it fits. If it doesn't, just take it right back in the store and return it. You'll probably have to do that when your kids can stay home, though.
Good luck!