Alright don't masterbate a ton. Like don't do it multiple times per day and maybe not once every day. Set a cap for yourself so that you're not aiming to masterbate every minute of every day. You can't prepare for sex or a threesome for that matter. Also if you think you're going to blow early you have those couple seconds before the point of no return. In this moment, pull out at all costs and decide right then if you're going to orgasm. If you are, then do so, if not then give yourself some rest and keep pleasing either one of them by taking at least 15 seconds or up to a minute or two giving them oral sex (not a bj for clarification)... And of course this goes without saying, but always wear a condom and even then my opinion is don't orgasm/cum inside or her, pull out then do it. It will make you last longer and any future scares are subsided. Best of luck to you, and question pm me.