The official website for The Settlers of Catan

Question about Base Game Trade - Is a “triangular trade” permitted? On player A’s turn, are player B and player C allowed to trade with each other if afterwards one of them uses the obtained resource for trading with player A?

Answer

No. Not only is a triangular trade forbidden, it is also unnecessary, because player B and player C each trades individually with player A and, thus, may trade whatever resource he has – provided that it meets the requirements of player A.