The trio, aged 21, 24 and 28, were arrested in southeast London on Saturday, three days after the 25-year old soldier was hit by a car and then attacked with a knife and a meat cleaver in Woolwich, Sky News reports.
They have been bailed to return to a south London police station, the Metropolitan Police said.
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.
Counter-terrorism officers are also continuing to question a 22-year-old man in Highbury Grove, north London, who was arrested on suspicion of conspiracy to murder.
Meanwhile, anti-terror police in Nairobi have confirmed that Adebolajo was also arrested in Kenya in 2010.
Adebolajo appeared in court in Mombasa suspected of leading a group of Islamists trying to join the al Shabaab group.
Anti-terrorism unit chief Boniface Mwaniki said Adebolajo was believed to have been preparing to train and fight with the al Qaeda-linked Somali militant group.