If you could measure your backseat, we could likely be able to recommend some seats. I know that Baby Trend
infant carriers (they are at bigger targets and babies r us) have a small base and the seats aren't too terribly wide.
I have a
safeseat sitting in my house right now (I purchased it for a friend) and it is 19" wide at it's widest point, so there goes 38" with just the base and the
marathon is almost 20", so if your backseat is around 58" wide, you may be able to get them in there.
As for the 3rd row in the Volvo, check the manual and see if you can have seats in that location. If you can, if the 3rd row splits, you could install one seat in that location, then your other 2 in the 2nd row. I have known many people to use their 3rd row with only one seat and put their child in through the back of the vehicle!!