The latest release came as negotiations reached what police said was a critical stage.
Some 157 hostages remain on board the jet, which remained standing on the tarmac as darkness fell.
Security chiefs said they were not aware of any demands made by the hijackers or of any political motives.
Supplies of food, drink and medicines have been delivered several times to those on board since the jet landed in the UK at 0200 GMT.
As the negotiations continued, five hostages - one woman, two children and two men - left the aircraft shortly after midday.
Three hours later two women and a man were set free.
Four of the five, a man of 26, a woman of 17 and two children, were from the same family, said police. The five - thought to be Afghan - are being interviewed by police.
The hostages indicated they had been treated well on board.
Officers did not give details of the terms of the releases, and said they were unaware of any demands made by the hijackers, other than for food, water and other supplies.
Water talks
Police said they wanted to ensure the hostages remained as comfortable as possible in the circumstances.
Essex Assistant Chief Constable John Broughton said: "We are seeking to enhance and improve the welfare of those on board in the hope that we can achieve a successful resolution."
Earlier in the day, a generator was supplied to provide electricity to the plane and medication was supplied for one passenger with a kidney complaint.
Talks were continuing into Monday evening over delivering fresh water and emptying the aircraft's toilets.
Hot food including fish and bread were delivered to the plane, said Mr Broughton.
Police and security experts spent the day in communication with the hijackers and watching the plane closely.
SAS soldiers were also believed to be part of the operation.
Flight delays
The waiting was also uncomfortable for passengers at Stansted, whose flights were delayed because of the hijacking.
For much of the day, airport chiefs operated only a skeleton service and many flights were diverted to other UK airports but by evening, up to 90% of flights were operating.
Stansted, which has seen several similar incidents in the past, has well-rehearsed procedures for dealing with hijacks.
But airport bosses want a review of the policy of directing hijacked planes there, saying the decision was taken when it handled far fewer passengers.
'Storm the plane'
The Boeing 727 was hijacked on Sunday during an internal Afghan flight.
It landed twice in Central Asia, where at least 10 passengers were released and the plane refuelled.
Click here to see the hijackers' route
Several hours later, it stopped in Moscow, where another 10 passengers were freed.
The hijackers' motives were unclear, but one report, from Pakistan, said the group wanted the release of a politicial prisoner.
In Afghanistan, the ruling Taleban movement said that while British authorities were free to negotiate with the hijackers, the Taleban itself would not give in to their demands.
Akhtar Mohammad Mansoor, the Taleban's minister for civil aviation and tourism, said the UK authorities should end the hijack by storming the plane.
"They can do it as they have past experience. They can separate the crew and the passengers and bring the hijackers to their knees. If not, then they should allow us to do it," he said.
Mr Mansoor said the Taleban did not know who the hijackers were, or their demands, and said it would not negotiate.
Police said the Foreign Office was talking to other governments about the crisis.
Also:
Hijack leads to travel chaos
Who are the hijackers?
Negotiating with hijackers
Stansted's hijack history
(click here to return)
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