The three patients were announced to have recovered after treatment at the National Hospital for Tropical Diseases.
The 12-year-old boy patient returned from Europe last month and tested positive for the novel coronavirus on June 19.
The other two recoveries are a 26-year-old man and a 36-year-old woman who got back to Vietnam from Bangladesh on July 3 and were confirmed as patients three days later.
Each has tested negative twice. All three will remain at the hospital for medical monitoring over 14 more days to ensure none experience a relapse.
Vietnam has recorded 381 Covid-19 patients so far, with only 25 still under treatment after 356 have recovered.
The nation has gone 91 days without a single case caused by community transmission, and recorded no Covid-19 death.