The ambush took place in Birgunj district near Bindhibasini village, south of Kathmandu on Monday, when police mounted an operation to rescue a businessman kidnapped for ransom by the rebels last week, a police spokesman said.
"The incident occurred when the police van tripped the ambush set up by the rebels," home ministry spokesman Gopendra Bahadur Pandey said.
"A hectic search has been launched to locate the rebels involved in the incident," he added.
Police superintendent Madan Khadka told reporters the policemen were killed before they could take any action against the rebels.
In another incident, two Maoists were killed in a clash with the security forces on Sunday night at Rupendahi, southwest of Kathmandu, police said, without giving details.
The Maoists have been fighting for a communist republic since 1996 and the uprising has so far claimed more than 9500 lives.