When you make your bid on Priceline you must agree (by making a specific indication) to their legal terms in line with the offer you make. They specify that you are bidding for a room at a rate for up to two people. Bringing in a third violates the terms of the bid. If you do receive a room that could accommodate three, you are free to ask the hotel if they would allow the third person. They could say no or charge a fee or allow it. You could also see if you can get away with it without telling anyone and accept the risk. My recommendation would be to bid for two rooms at a good price.