(1) If what A says is true, then what B and C say is false. In this case, it should be that B can swim and A can swim, but only one can swim, so it is contradictory.

(2) If what B says is true, then what A and C say is false, then what A and C say is contradictory, so it is wrong.

(3) If what C said is true, then what A and B said is false. In this case, it should be that A can't swim and B can swim, which is not contradictory, so the final result is that B can swim.