The three people were among nine hospitalized during last weekend's Hard Summer Music Festival in Fontana, San Bernardino County sheriff's spokeswoman Jodi Miller said yesterday.
More than 146,000 people attended the Saturday-Sunday outdoor event, according to organizers.
The victims were identified as Derek Lee, 22, of San Francisco; Alyssa Dominguez, 21, of San Diego; and Roxanne Ngo, 22, of Chino Hills, the San Bernardino County coroner's office said.
"We were deeply saddened to learn about the deaths of three people who attended the festival this weekend," Hard Summer spokeswoman Alexandra Greenberg said in a statement.
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"Our sincerest thoughts and condolences are with their family and friends."
Hard Summer was moved to Fontana's Auto Club Speedway this year after two young women died of drug overdoses at last year's event at the Los Angeles County Fairgrounds in Pomona.
The parents of 19-year-old college student Katie Dix recently sued event promoter Live Nation, Los Angeles County and others over Dix's death, which authorities blamed on multiple-drug intoxication.
The defendants have declined to comment on the lawsuit.
Ecstasy is a popular drug at raves, where it's used to enhance the musical experience. It can lead to dehydration, however, causing concern among medical officials that its use in hot, crowded quarters where people sometimes dance for hours can lead to collapse and sometimes death.
Temperatures in the Fontana area, 50 miles east of Los Angeles, have been in the 90s.
Some venues have banned raves amid mounting concern about overdoses.