I'm having the same exact problem with my RIA 10mm (4.25). The slide was locking back after the 3rd round with 3 different mags. The only thing I could think of was maybe the slide stop was moving up on its own or I was moving it with my offhand thumb under recoil. It's weird that it only happens after 3rd round though. I replaced the slide stop with a lower profile Nighthawk custom slide stop. It dropped right in. I haven't had a chance to shoot yet but I will report back when I do.