The three - all Britons - had been arrested early on Sunday, but officials did not say if their arrests were related to a massive hunt for the bombers who killed at least 49 people on London Underground trains and a bus.
A police spokesman said that the three men, who were not identified, were questioned and released.
Police had disclosed the arrests earlier during a briefing on their investigation and had cautioned against linking the detentions to the Thursday explosions.
"Three people have been arrested under the Prevention of Terrorism Act at Heathrow airport," Scotland Yard Deputy Assistant Commissioner Brian Paddick told a news conference on Sunday.
He did not say whether the suspects were entering or leaving the country.
Paddick said it would be "pure speculation" to link them to last Thursday's bombings on three underground trains and a bus, from which police have recovered 49 bodies and are still retrieving more from a tunnel below King's Cross station.
He later told Sky Television: "My understanding is these are reasonably routine arrests under the Prevention of Terrorism Act... there is no connection that we know of at this stage."
Britain has detained more than 700 people under anti-terrorism laws since the 11 September 2001 attacks on the United States.
Of these, around 120 have been charged with "terrorism" offences and another 135 under other legislation.
The centre of Birmingham was
evacuated after a security alert
Police have so far not reported any arrests directly linked to the attack on the capital, which the government says bears the hallmarks of al-Qaida.
They have been at pains to stress that the inquiry will require slow and meticulous work, including the gathering of forensic evidence from the four bomb sites, three of which are in underground tunnels.
Paddick said police had received important information from members of the public, who had placed 1700 calls to a special investigation hotline.
"A considerable number of these calls are proving to be very, very valuable to us," he said.
Police say the three underground bombs went off almost simultaneously, making it more likely they were detonated by timers, rather than bombers. That means the bombers may still be at large and could strike again, they said.
Meanwhile, a huge overnight security alert in Britain's second city of Birmingham kept Britons on edge on Sunday.
Officials are still searching for
bodies trapped underground
Police evacuated 20,000 people from the city centre on Saturday night and carried out four controlled explosions on a bus. They found no bombs but said the drastic measures were fully justified.
"The threat that we responded to yesterday was very specific," West Midlands police Chief Constable Paul Scott-Lee said on Sunday.
"It was specific about the time and also the locations... The people of Birmingham were in danger last night."
Meanwhile, in London, anxious relatives continued to scour hospitals in search of loved ones missing since Thursday's blasts - the worst peacetime attacks on the British capital.
Walls, bus stops and telephone boxes close to King's Cross station, scene of the worst blast, were covered with photographs of missing people and appeals for information about them.
At least 49 people were killed in
the four blasts
Well-wishers have left hundreds of bouquets of flowers outside the station, many accompanied by messages testifying to London's multi-ethnic and multi-national mix.
"Christian, Muslim, Jewish, Hindu, Sikh, Buddhist. We are all Londoners," read one message scrawled on a Union Jack flag.
"Our prayers are with you. Keep the faith. From all South Africans," read another written on that nation's flag.