I'm not sure what type of van you have nor what brand name of car seats you have.
I have the Town & Country with stow-away seats, also with middle row capin chairs and bench in back.
My childrens ages are 4, 2, and 1. My 4 year old sits in the Britax Regent, 2 year old sits in the Graco Nautilus 3-in-1, and the 1 year old is in the Britax Marathon.
What I have done is put the Regent and Graco on the bench seat. The Regent is on the right and Graco on the left.
I used my D-Ring Connector Strap, which is actually from my Britax Marathon seat but you can buy it at $5. This allows me me to be to use the Top Tether Hook strap, it's that long strap that helps to anchor the top of the child seat, on the Graco Nautilus.
What I did was found a metal spot of the bench seat itself that I could loop the D-Ring Connector around and then hook the tether to. I did this because on my bench on the left side it's actually like a small seat and you can fold just that small section down if needed and it doesn't have the actual small metal bar on the back of the bench seat itself for hooking a Top Tether Hook to.
Then I did the stow away on the captain chair directly behind the drivers seat. I then put the Britax Marathon on the other middle row captain chair (behind the passenger seat). There's a bunch of hard plastic on the floor for the stow away seating so what I did was took a bathroom rug, because of it's plushness and rubber backing to keep it in place, and layed it on the floor for my knees and to keep the debris to a minimum.
Sometimes I think that maybe I should seperate the two older ones but really they only get on each others nerves when they are more tired so any way so it's really not that big of a deal.
Hope this helps.