Scotland Yard said a 25-year-old man who was arrested in Newport, South Wales, on Tuesday September 19 and a 30-year-old man also arrested in Newport, on Wednesday September 20, were released on Tuesday afternoon.
A 20-year old man arrested in Cardiff on Monday was also released.
Seven people have been arrested as part of the investigation into the attack on a packed District line Tube train.
One person, Ahmed Hassan, has been charged, and the other six have been released with no further action.
Police said searches are continuing at one address in Cardiff as the Met’s Counter Terrorism Command continues to investigate the attack on Friday September 15, while a search in Surrey has been completed.
Hassan, 18, an Iraqi asylum seeker, appeared at Westminster Magistrates’ Court on charges of attempted murder and using the chemical compound TATP to cause an explosion that was likely to endanger life.
He allegedly bought parts to make the improvised explosive device online and left the train at Putney Bridge before the device partially detonated at the next station.
Witnesses described how a fireball erupted inside the carriage, leaving 30 people injured, including one woman with serious burns.
Hassan, of Cavendish Road, Sunbury, Surrey, was arrested at the port of Dover in Kent at 7.50am on Saturday September 16 by counter-terrorism officers.
The teenager, whose parents are understood to have been killed in Iraq, was in the foster care of Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.