Older station wagon - the real full size kind, Buick Roadmaster was the last of them.
Other than that, you're stuck with a "crossover" that has a third row of seats - Mazda CX-9, GMC Acadia, Saturn Outlook.
The current generation of station wagons, like the Volvo, won't let you put 3 child seats across the back, unless the one in the middle is an infant seat facing backwards. They just aren't wide enough.