We've had a 740 and now a 940 wagon since 1992, before kid #3 came along. At one point we had an infant seat, a toddler seat, and a booster in the back. It was tight, and I don't think we could have managed two Fisher-Price toddler seats (the best!) and a booster seat. OTOH, there is no way all that stuff would have fit in a Corolla or Tempo!

The third seat really helps relieve the pressure, but there are limitations. You end up with no separate cargo area, and the kids in the "way back" are quite isolated. The novelty wears off on your own kids fast, but visitors love it!

Check out the integral booster in some 740 and all(?) 940 wagons. Maybe you can put the biggest kid in that and liberate enough space for the toddler seats. In a pinch, you could get the rear seat back from a 940 wagon at a scrap yard and install it temporarily in a 740. Who cares about the colour, it's going to get trashed by the kids and dog anyway!

Our kids are now eleven, nine, and seven and we have no extra seating appliances installed. The 940 wagon is entirely adequate now. Ironically, the bigger your kids get, the more space you have as you retire seats! We even have a dog now, but it's a small one that is OK in a small carrier in the back or on a lap. Minivan successfully avoided!