Gallo was found guilty in Orange County Superior Court of three counts of second-degree murder in the 2009 collision in Fullerton.
He faces 50 years to life when he is sentenced Dec. 10.
Gallo, 23, who had been drinking in a bar in Covina, had a .22 blood-alcohol level at the time, which the Orange County (Calif.) Register said was three times the legal limit in California.
Adenhart, 22, and three friends had been headed for a nightclub after he had pitched six innings for the Angels in nearby Anaheim. They were crossing an intersection on a green light when they were struck by Gallo's minivan.
One person in Adenhart's car was seriously injured as was a passenger in Gallo's vehicle.