All I can say is dry fire, practice mag changes and get your gear straight then do it all again. And remember you may think you are prepared but you're not. It never fails, something will go wrong.
Knowing you were wanting to shoot 3Gun you would have been better served w/ a longer rail and lighter barrel. Trust me on this one. I may come off like a jerk but I'm only trying to help through my mistakes.
Also make sure you have a good/fast holster and magazine pouches. Position them in a natural manner. I usually let me arms hang then bring them up like I was drawing or grabbing a mag then place the pouches and holster there.