Two boosters and a car seat is tricky, because you have to reach around those boosters everytime into the tight spot to buckle the kid.
Three carseats are easier, because you only have to squeeze them all in there one time, then the rest of the time you are buckling on top of the seat.
I know when our third was born, and my 5 and 6 year olds weren't able to buckle thier own seatbelts anymore, and my knuckles were scraped because the buckles were so hard to get to, it was very difficult. I moved the oldest out of a car seat, and then scooted the infant seat as far onto his side as possible, and that helped make buckling the other booster a little easier.
Could your older kids move back into seats with 5 point harnesses? I think that really does make it easier to find a car that they don't only fit in, but that are not too hard to get kids buckled into.