Adrien Broner just learned the hard way ... if you're going to record a threesome on your phone -- DON'T LET IT GO PUBLIC!!!
A Nevada court just ordered the boxer to fork over $175,000 (plus other fees) to one of his co-stars from a 2013 romp after the footage mysteriously surfaced on World Star Hip Hop.
Broner had argued that he didn't intentionally leak the video (which was shot with the consent of his 2 partners) ... but he had lost the phone he shot it on and someone else posted it.