The second case was a University of Southern California student who traveled to the East Coast over Thanksgiving, county health and USC officials confirmed Monday.
The USC student is fully vaccinated and experienced mild symptoms and is in isolation, according to the county.
The third case was announced late Monday. The person was described only as someone who had recently traveled from West Africa and is fully vaccinated.
The person had mild symptoms and was self-isolating. The individual's known close contacts were also described as fully vaccinated and testing negative.
Health officials said they're working with USC along with people he or she may have been in contact with in the county, all of whom are currently in quarantine and are getting tested for the virus.
USC says the student had not attended classes or participated in organized campus activities during the infectious period.
MORE | Expert explains omicron variant's potential impact on pandemic
They add that based on the student's travel history, it is likely that the infection was acquired outside of L.A. County.
"While we are still determining the transmissibility and the severity of omicron, I encourage residents and travelers to take additional steps to protect yourself and those around you by getting vaccinated or boosters, tested if you feel sick or are a close contact, and wearing your mask," said Dr. Barbara Ferrer, the county director of public health.
"Layering on as many protections as possible will give us a better opportunity to slow the spread of this potentially dangerous variant as we prepare for holiday gatherings and a potential winter surge."
The county's first case of the omicron COVID-19 variant was reported on Dec. 2. That person returned to L.A. County after traveling to South Africa via London on Nov. 22, according to health officials.
That person was also fully vaccinated. Health officials added that the infection was most likely travel-related.
READ MORE: First case of omicron COVID variant reported in Los Angeles County
The first U.S. case of the variant was reported in a San Francisco resident.
As the omicron variant continues to take hold in California, Los Angeles County also announced new rules to increase safety measures at local nursing homes.
From Dec. 15 to Jan. 31, 2022, any skilled nursing facility resident, employee or contractor who may come into contact with residents must be tested for COVID-19 once a week, regardless of vaccination status.
Anyone who enters a skilled nursing facility, regardless of vaccination status, must provide proof of a negative PCR test within 72 hours prior to entry or a negative antigen test within 24 hours of entry.
Anyone who shows documentation of recovery from COVID-19 within the prior 90 is exempt from the weekly testing and the test for entry.
The county reported two additional deaths and 1,103 new cases of COVID-19 on Monday.
In total, 27,248 people have died from COVID-19 since the start of the pandemic and the county has seen more than 1.5 million cases.