E. -
I feel your pain! I had a three year old when I gave birth to my twins. Three kids in carseats wasn't easy! We purchased a Honda Odyssey. The kids are now 7, 4 & 4, and I'm still driving it. Love, love, love it.
Lots of versatility in how your configure the actual seats in the car with the Odyssey, and that helped. You can actually take out the center console thing and shove the two middle row seats together, allowing easy (easier) access to the back row. We put the older girl in that seat and you just have to reach through and help with those buckles. As she got older, she needed less and less help. Now all three are in boosters and things are definitely easier! We've now removed one of the two center seats entirely from the van. One girl sits in the remaining center row seat and the other two in the very back bench. The floor where the seat was removed is completely empty, allowing super easy access to everything. The girls can step in, walk to their seats - easy. Lots of work ahead of you, but it gets easier. When you shop for your minivan, make certain they show you EVERYTHING in terms of the versatility of the seat configuration. I'd recommend the Honda! Good luck.