The alleged assaults happened sometime Friday evening when a group of acquaintances -- the three teen boys and two girls -- were in the Alvarado area of the park, according to Lt. Jon King.
The suspects gave alcohol to the girls, ages 14 and 17, and at some point the girls became incapacitated and passed out, King said.
He said that when they awoke the next day, they discovered that they had been sexually assaulted. With the help of family members, both sought medical treatment and reported the assaults to police.
Richmond police on Wednesday assisted in the arrests of all three suspects, who were taken into custody at their homes in Richmond.
The suspects -- one 15-year-old boy and two 17-year-old boys -- were each arrested on suspicion of multiple crimes, including rape, rape in concert with others and conspiracy.
All three boys, whose names are not being released because of their age, were taken to Contra Costa County Juvenile Hall in Martinez where they remained Thursday.
The alleged assaults remain under investigation. Anyone with information about the case is asked to call the East Bay Regional Parks Police Investigations Unit at (510) 690-6555.