When you are caught between the fear of rejection and the regret of never trying, remember that the latter is the price you pay for letting fear make your decisions.
Good communication is key to having a great experience. Many people have had less-than-pleasant experiences with threesomes, and that may well be because they failed to communicate properly beforehand.
Peter, however, handled this very well. He had already talked things through with his partners and knew what they truly wanted.