To be totally fair; The guys I saw lose their pistols were not using race holsters.
One guy was shooting his rifle prone. Stood up and his gun was laying there on the ground. DQ.
Second guy was flipping his shotgun around to quad-load. Shotgun stock hit his handgun and knocked it out of the holster. DQ.
Third guy's gun didn't actually leave the holster. His holstered handgun pulled right off of his belt when it caught on a barricade. DQ.
Whatever holster you use for 3-gun; Make sure these things can't happen to you.