The three positive cases were detected over the weekend in Ho Chi Minh City and confirmed as omicron late on Tuesday, the Tien Phong Newspaper reported, citing health authorities.
The Southeast Asian country had previously detected more than 70 cases of the highly transmissible variant among quarantined people entering Vietnam from overseas.
"The community needs to raise their vigilance against omicron, not gathering at crowded places and getting fully vaccinated," the Ministry of Health said in a statement last week.
Overall, Vietnam has recorded more than two million COVID-19 infections and nearly 36,000 deaths.
More than 74% of the country's population of 98 million have received at least two vaccine shots, according to the health ministry.
Vietnam has also launched a booster vaccination drive, with authorities pledging that all adults should get the additional vaccine shot by the end of March.