The Marines were driving through Dana Point around 2 a.m. on Tuesday when their car slammed into a palm tree on Golden Lantern and Terra Vista.
Authorities said the driver of the Dodge Stratus is the sole survivor of the crash. They believe he was driving too fast and lost control.
"We believe the driver lost control of the vehicle, striking the curb and then striking the tree," said Jim Amormino, spokesman for the Orange County Sherriff's Department. "It took approximately 20 minutes to extricate the victims from their vehicle. Two were pronounced dead at the scene, one died at the hospital."
Amormino said the driver, in his 20s, is at the hospital in critical condition. He is in intensive care at Mission Hospital.
They were all active-duty Marines stationed at Camp Pendleton.
An investigation is under way to determine the cause of the crash.
"We are looking into the possibility whether alcohol was involved in this accident," said Amormino. "At this stage of the investigation it appears that unsafe speed is the primary cause."
A memorial at the scene of the crash continued to grow throughout the day.
"I hate to see it, it's way too young," said one person paying his respects. "When I saw this just a few minutes ago I had to come up and leave some flowers on Valentine's Day.
The names of the Marines and their home towns were not released pending notification of next of kin.