hi i too had the same problem. needing a commuter car that can carry two adults and two kids. i have a three yr old and a 8 mn old. i was riding around in an 91 civic before but needed more room.
i just bought a gsr sedan, and it works well for two car seats in the back. i don't know if it would fit three though. i use the britax roundabout, btw. sometimes my wife sits in the back with the baby when we ride back home due to him crying sometimes. we commute like a 1hr and half one way. traffics sucks in the I.E. when she rides back there she has a hard time putting on the seat belt due to the second car seat sitting in the middle. she is petite in size, 5', 110lb.
you may need to upgrade to a bigger car.....