I say, and with God's success: Yes, such a sale is permissible, and the seller's offer implies that the price of the three in bulk is the same as the price of the two if sold individually. Our Sheikh Al-Othmani said in the jurisprudence of sales (2: 774): "What is announced by the seller is that if the buyer purchases two of something, then the third is given to him for free. It seems that this is not an increase in the sold item, but rather a sale of the three together; because the contract was made for the three from the beginning. The meaning of the seller's statement that the third is free is that the price of the three in bulk is the same as the price of the two when sold individually," and God knows best.