If you select A and C, then you should be prepared to sit in A and C. If you are hoping that the person who is in B is kind enough to switch seats with one of you, I would think that you'd make the offer as attractive as possible by offering them either seat.

I think most people would be happy to get out of the middle seat, but some people might find it irritating that you assume that they will give up their seat for you while acting as if you're doing them this big favor.