The hostages were released several hours later, after pressure from both Palestinian authorities and militant organizations.
Five gunmen seized the foreigners — an American, a Briton and an Irishman — as they were returning to the home where they were staying around midnight, the officials said.
Their identities were not released. No reason for the abduction was immediately given.
Five foreign women accompanying the men were left unharmed. They refused to speak to reporters.
Police and militants from the Al Aqsa Martyrs' Brigades, which is affiliated with Yasser Arafat's Fatah movement, were searching for the kidnappers and hostages in the Balata refugee camp. The kidnappers were believed to be from a splinter group of Al Aqsa.
The volunteers came to Nablus more than three weeks ago, apparently for missionary activities, the officials said.
The abduction took place in an upscale part of Nablus, where restaurants stay open late into the night and where young people often meet.
On July 16, four French volunteers were kidnapped in the southern Gaza Strip city of Khan Younis but were released within a few hours.
That abduction was linked to political turmoil and infighting within the Palestinian Authority, and was one of a series of incidents that led to a confrontation between Palestinian leader Yasser Arafat and Prime Minister Ahmed Qureia.
Arafat and Qureia patched up their differences, however, after Arafat yielded some control over the Palestinian security forces.