The trio, aged 21, 24 and 28, were arrested in southeast London on Saturday, three days after 25-year-old Lee Rigby was hit by a car and then attacked with a knife and a meat cleaver in Woolwich.
They have been bailed to return to a south London police station, the Metropolitan Police said in a statement.
Michael Adebolajo, 28, and Michael Oluwatobi Adebowale, 22, who were arrested on suspicion of murder at the scene of the killing on Wednesday, remain under arrest.
Meanwhile, the British Foreign Office confirmed that Adebolajo had been arrested in Kenya in 2010. He was arrested with five others, and later deported, which is common in Kenya when involvement with terrorism is suspected.
The Foreign Office in a statement said: "We can confirm a British national was arrested in Kenya in 2010."
It provided consular assistance, as is normal for British nationals.
The two suspects arrested at the scene of the killing remain in hospital and will be formally interviewed when it is possible to do so, police said. They were shot by police when they charged towards armed officers in Woolwich.
The arrests came as Downing Street confirmed the launch of a new terror task force to crack down on extremism.
Prime Minister David Cameron, who is on a family holiday in Ibiza, has also said that the parliamentary Intelligence and Security Committee (ISC) will carry out an investigation after it emerged that Adebolajo and Adebowale were known to MI5.
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