You can easily get 3 child seats in the back of a V70 and for 15k I'd go estate all the way. You'll get something newer, better to drive, more comfortable, safer and much better on fuel.
I have a V70, and am not sure you could get three child seats in teh back. As mentioned above, might have to have one adult in the back.
Newer? Maybe
Better to drive? Maybe, but only marginal. The XC90 is very good, and you're hardly caring about rad to teh power of sick driving skillz with three kids in the back, are you?
More comfortable? Same seats as a V70, so the same comfort!
Safer? Again, debatable. Not much in it either way, I would say.
Much better on fuel? Better maybe, but not much better.
Plus, as the kids grow, the extra seats in the back will be great.