Three men have been found guilty of gang raping a teenage girl at a western Sydney house party after police found footage of the assault on a GoPro camera.

A jury at the Downing Centre District Court on Tuesday found Tristan Carlyle-Watson, 26, Kurt Stevenson, 26, and Andrew Waters, 25, guilty of aggravated sexual assault in company.

During the trial, crown prosecutor Sharon Harris said the 16-year-old girl was drunk and believed she had been drugged before the assault at the St Clair house in May 2015.

In a 17-minute-long video, Ms Harris said a group of men sexually assault the girl, "sometimes... simultaneously, sometimes taking turns."