The incident appears to be a result of people not heeding warning signs posted near the falls. The Merced River has been treacherous this year due to record snow melt.
A Yosemite Search and Rescue unit identified them as Homiz David, 22, of Modesto; Ninos Yacoub, 27, of Turlock; and Ramina Badal, 21, of Manteca. All were members of an Assyrian church in Turlock.
The victims were part of a group of 10 hiking in Yosemite. Witnesses say part of the group crossed a barricade to pose for pictures near the river's edge. Other hikers warned the group to come back. A second group of hikers, according to witnesses, crossed the river and made their way to a rock in the middle of the river.
"One of the males lost his footing, started to slide down. The second male tried to rescue him and also lost his footing. Then the third person, the female did, and unfortunately it was a chain-reaction of events and all three were swept over Vernal Fall," said Yosemite National Park Ranger Scott Gediman
"I saw two that were holding onto each other and one that was alone, floating through the current," said Dean Sabo, who witnessed the incident. "The reason why I looked was this woman was screaming and running along the edge of the water."
"I saw the man's eyes as he was going over the waterfall, and that was devastating," said eyewitness Jacob Bibee.
Members of the St. Mary Assyrian Church in Tarzana held a candlelight vigil for the hikers Wednesday night. Those who knew them say they were role models who did not normally take risks.
"We're just praying to God that they come back to their houses safe," said Rameil Eaywazi of St. Mary Assyrian Church.
Part of the Mist Trail was closed while rescue personnel searched for the hikers. The trail is used by 1,500 people a day.
Rangers say it could be weeks before the bodies are found.
The tragedy brings the number of people killed in water accidents to six in Yosemite this year. Two hikers drowned in the Hetch Hetchy Reservoir on June 29, and a hiker slipped and fell into the Merced River on the Mist Trail on May 13.
The Associated Press contributed to this report.