The accident occurred at around 5 pm on August 25 when a boat carrying four people suddenly capsized while travelling along the Linh Nham River, according to authorities in Dong Hy Commune.
One passenger, who was wearing a life jacket, managed to drift safely to shore. The other three occupants remain missing.
The section of river where the accident occurred lies near the boundary of three localities: Van Han Commune, Dong Hy Commune and Linh Son Ward in Thai Nguyen Province.
Following reports of the incident, authorities from the three communes and wards mobilised emergency personnel and local forces to search for the missing victims.
However, by 10.30 pm on August 25, rescue teams had only managed to recover the overturned boat, while the three missing individuals had not yet been found.
Officials said the victims were between 23 and over 40 years old and were not residents of Dong Hy Commune.
Authorities have not yet been able to contact the survivor who reached shore, making it impossible so far to determine the cause of the accident or the purpose of the boat trip.
The search operation comes as rivers across Thai Nguyen have experienced rising water levels in recent days due to heavy rain and storm-related flooding.
Several residential areas along the Cau River remain inundated, with some transport links still disrupted by floodwaters.