There are not many 3 in 1 seats out there that actually do all 3 things well. You are MUCH better off purchasing a good convertible seat that will get them to appropriate booster age then purchasing a dedicated booster.
Most convertible seats allow for the seat to be more upright allowing more room than infant seats, so if you fit two infant seats you should have no trouble whatsoever fitting two convertible seats.
The britax next generation models (marathon 70, boulevard 70, roundabout 55) all fit very well in smaller cars and will get most kids to 2.5-3yrs rear facing and 4.5-5 forward facing. This is what we have for our Toyota Yaris. If you buy online you will find much better prices than in store, but you can still try them out at Babies R Us if you'd like first.
Evenflo Triumph Advance is another option offers a slightly taller shell and harness height meaning more time before it is outgrown. I don't have personal experience with this seat in your car, but I have found that overall it is a pretty easy seat to install and use.