C.W.
There is a newer product from Britax - the Frontier - see britaxusa.com. Its a 5pt. harness that will still work with a bigger child, and then you can eventually convert it to a booster when the child is old enough. And it goes to 100lbs, so the kid will almost be driving themselves by the time they are done with it ;)
Though I must tell you that it sounds as though 3 children restrained in the back seat might be tight in any car. In fact, I've never heard of anyone who had 3 car seats across in back seat....everyone I know in this situation has had to take a deep breath and invest in a newer, larger vehicle. Might be able to find some really great used car deals...good luck!