Is it possible to fit three children in a back seat and which car seats will work?

I have three kids ages 5,4 and 3. I am trying to find a way to fit them all into the baby-sitter's car, so that she can get them to school. Does anybody know of a narrow backless booster, or some combination that might work. I thought that maybe 2 Sunshine Radians and a booster in the middle. I want to make sure that my kids are safe, but we would prefer not to have to purchase a third car. Any thoughts?