The 2003 Nissan Maxima has 53.7 inches of rear hip room, not too shabby, that's wider than the third row of my minivan.
Knowing your 3.5yo's exact height and weight would be immensely helpful in choosing a seat that will last her enough time, harnessed, while still being as inexpensive as possible. Also, what seat is she currently riding in, and how is she fitting? Where are her shoulders in relation to the top slots?
Off the top of my head, I'd go with a Sunshine Kids Radian 65 (about 14.5-15 inches wide at the base, 17 inches wide at the widest point, the shoulder area, harnesses up to 65 pounds) for your 3.5yo and Graco SnugRides or Baby Trend Latch Locs/Flex Locs for the babies (SnugRides are 17.5 inches wide w/ base, the Baby Trend seats are supposed to be very narrow at 16.5 inches). The Baby Trend seats can sometimes be a bit more difficult to install with the base if using the seatbelt instead of lower anchors, and usually when trying to fit three-across you'll want to use seatbelts anyway, because LATCH installs usually pull the seats a little more toward the middle of the vehicle. But the Baby Trend seats have a slightly taller seat shell (so it'll last longer for a taller, longer-torsoed baby) and slightly shorter bottom slots (so it'll fit smaller/shorter-torsoed babies better). I'm not sure of the Chicco KeyFit's measurements, but if there's a chance your babies will be born/come home at less than 5 pounds, I'd strongly look into those (4 pound minimum weight, very good newborn padding, fits preemies well).
Radians aren't awful to use for newborns or young babies, they have pretty low bottom slots, but I'd hesitate to recommend them from birth for twins. They also can take up more space front-to-back when rear-facing, so you might want to use something else with a slightly shorter seatback for rear-facing once the infant seats are outgrown (because rear-facing as long as possible is safest, to the limits of the convertible seat, which is 30/33/35 pounds depending on manufacturer).
Anyway, I hope that helps some, and if you post back with your daughter's information it'll help with other recommendations.