We are in the same boat! Baby #3, living in rural Alberta and looking at a new vehicle and possibly car seats!
We have a Ford F350 extended cab long box, which back seat is quite wide and can easily fit 3 big car seats across (have done it before) but I don't want to use the truck on a day to day basis. We had a Hyndai Elantra but we sold it last month just before we moved and there was definately no room for a third carseat between my 4.5yr old and almost 3yr olds carseats anyways.
Hubby and I have been sharing the truck since we moved which has been an adjustment lol but I need something more practical for commuting to the city, etc with the kids.
As for carseats, we are considering the radian RXT as it so narrow, you can fit 3 across most midsizes but I need to do more research on these car seats and possibly consider something else for the 4yr old as he is very tall for his age.
We currently have Britax carseats that are great, kids love but they are so bulky and take up so much room, I think we would be almost forced into a minivan if we don't get new carseats.
I would love to hear everyone elses thoughts too :)