hi, I have a 1999 Saab 9-5 Wagon and I've had two forward facing child seats and one rear facing seat in it (all three are Britax car seats) and now I have two booster seats and just one forward facing child seat and both configurations worked fine. You need a car with 3 seatbelts in the back and then it does need to be wide enough. With the Saab I fit the 3 in easily with inches still free on the sides by the doors.
I can fit them in any sequence so I would change abit: between having the baby in the middle where they say it is safest and moving her to the side where I don't break my back lifting her in.
I love having the wagon -I can't live without the cargo area: I can have the kids all sitting in the cargo area as I load stuff up at Target/Costco/other, I can lay baby down to change diapers, we eat snacks there, I sit on the 'tail gate' in the parking lot, giving baby bottle while the 2 toddlers play games in the cargo area - safely waiting for me to get done and having fun doing so. I'll never go back to a sedan with a trunk.