The short version is that just before and during orgasm you tighten your kegel muscle, or pubococcygeus muscle, the same muscle you use to stop the flow of urine. At this time you should stop or slow any stimulation until it passes. If successful, you won't have ejaculated and should be able to continue.

The longer version is here. When I was still doing this regularly I could get to three but usually just did two. I was successful right off the bat, but I never exercised the muscle enough to do it many times in a row, and eventually I stopped altogether. Whenever I try it now, I either fuck it up or it's just too tempting to let go. Part of this is because I never think to try until the last minute.