Jamie Maclennan attacked the girls, including a 15-year-old, at the birthday party at a miners club.
Maclennan, 25, lured the youngest victim to an isolated toilet cubicle at the back of the stage and forced his hands down her pants.
A second girl, aged 17, was also assaulted by Maclennan during the party when he grabbed her arms and forced himself on her.
He also slapped the bottom of third 17-year-old schoolgirl after she had asked him to pay her favourite song during the dance at Macmerry Miners Club, in Macmerry, East Lothian.
Maclennan, from Larbert, near Falkirk, had claimed he had consensually kissed two of the girls during the evening but denied assaulting them at the party in April last year.
But Maclennan, who has been restricted to non-clinical duties at Edinburgh’s Western General Hospital following his arrest, was found guilty of all three sex assaults following a five day trial at Edinburgh Sheriff Court last week.
All three teenagers gave their evidence to a closed court.
But Maclennan told the court the 15-year-old girl had come on to him by “dancing provocatively” in front of him while he was on the decks and that the pair had engaged in “flirtatious banter” during the evening.
He also admitted taking scores of selfies with the youngest girl while on stage and claimed he thought she was 17-years-old.
But prosecutor Kim Schofield said: “You took advantage of your position. You were a DJ at an 18th birthday party and you are 25.
“You lured her down a corridor on the pretense of giving her a business card. You took her in to one of the toilets and stopped her from leaving.
“You forced yourself on her, you kissed her and she repeatedly tried to push you off.”
Maclennan said he and the 15-year-old had been attracted to each other but he had not assaulted any of the three girls.
But a witness told the court she saw the 15-year-old flee back down the corridor in floods of tears following the attack.
Following the party all three girls made complaints to the police of being sexually assaulted by Maclennan and the NHS employee was arrested the following week.
Sentence was deferred.