I currently drive a Holden Commodore Wagon, not the current sportswagon one but the model before 05 VZ. It is basically based on the statesmen so same in length and width. I have 3 carseats no worries.

I have fitted a babylove capsule, safe n sound maxi rider and safe n sound hi liner booster with no worries. Then when DS grew out of capsule I had the Safe n Sound meridan in the middle (forward facing) and the maxi rider on one side and the booster on the other.

To be honest I think you will find your statesman is one of the widest across and would best accomodate 3 seats, think the only car that is good is the Ford Territory.