Well, first, depends where you need the narrow. The Parkway Slide Guard is very narrow all the way up, and the TurboBooster is a pretty close second. But, the Monterey is narrowest high-back I know of at the back/bottom (which makes it easier to buckle sometimes.) I might do one vest in the middle and two PWSGs or Turbos outboard (Turbos are cheap, too...) or you could maybe try a Monterey backless (in fact I think they're coming out with a version without the back) in the middle with two other boosters on the side, or even two Montereys and a vest in the middle for ease of buckling all of them... Mainly, I recommend going to a store that has a big selection of boosters, if you can, and trying different combinations in your car. But as a Corolla owner I think you do have a better shot with one vest in the mix and any two boosters than with three boosters, although I think with the right combination it could be done (especially if one was used backless.)