I don't have your problem (3 kids, 1 backseat) but I do have a very small car and I purchased this: SafeGuard Go booster seat. Which is quite narrow and will do for the 3 year old and then if converts to a booster. So far we really like it, it attaches to the car's LATCH system and has a soft back. It also has a travel case which is why we bought it. It is a bit expensive (we think) but if it lasts for the next 6 years (we bought this when we switched to a forward facing seat this month) it will be worth it.
Hope you find a way, Good luck, N.