The three volunteers with the British medical charity Merlin were among a group of between 10 and 12 people forced into the bush by rebels at Kalahun, Lofa County, in western Liberia. They are Sara Nam, 30, a midwife from Carmarthen, South Wales, David Heed, 26, a logistician from Wellingborough, Northamptonshire, and Dr Mike Roe, 33, from London.
The others included volunteers from Medecins sans Frontieres, Action Contre la Faim and a United Nations official.
Last week five British officers and some 35 other people were taken hostage in Sierra Leone.
The latest kidnappings coincided with Liberia's President, Charles Taylor, who backs fighting factions in Sierra Leone, claiming that dissidents from neighbouring Guinea were occupying Kalahun.
Jules Pieters, Merlin's director of operations, said of the hostages: "They are all experienced aid workers who are determined to help Liberian people in desperate need. We appeal to their captors to release them without harm immediately ...We are taking steps to ensure the safety of our other staff in locations around Liberia."
Meanwhile, three British oil company employees kidnapped in Nigeria were released yesterday, as negotiations continued over two other Britons still being held by gunmen.
David Shears, a helicopter pilot, was freed with a Nigerian colleague yesterday morning. In the afternoon, David Welch 64, an engineer, and Keith Miller, 47, were also freed. They were said to be unharmed and were being reunited with their families last night.
Mr Shears is an employee of Bristow Helicopters, working under contract to Texaco. Mr Miller and Mr Welch work for Niger-Benue, a transport company. The companies refused to comment on whether any ransom had been paid.
The Foreign Office said it had not been directly involved in the negotiations.
Mr Shears was taken with others eight days ago when an armed gang stormed an oil production platform. He was forced to fly his colleagues to a village. The other captives were released on Friday while talks continued over Mr Shears.
A spokesman for Bristow Helicopters said: "We are obviously delighted that this has ended with a successful conclusion to negotiations."
Mr Miller and Mr Welch were seized on Monday outside Niger-Benue's offices in East Warri. The area has beenracked by violence for two months June and night curfews were imposed.
Robert Msude, a spokesman for the firm, said the capture of the two men was linked to fighting between rival tribes in East Warri. He said: "Negotiations have been continuing and we had been waiting for the outcome of the discussions. The main thing was to find out the kidnappers' demands, and respond accordingly."
Kidnapping for ransom has been increasing in the Nigerian oilfields in recent years.Nine Britons were taken in the southern Delta region in three incidents this month.