The three, whose hooded bodies were discovered in separate isolated spots in south Armagh, were also accused in a detailed IRA statement of murdering a woman, the girlfriend of one, because she was on the brink of exposing their links with MI5 and the RUC.
A priest called to administer the last rites to one of the three said that the body was covered in marks, with blood on the hands and neck, suggesting that he may have been tortured.
The IRA said they had 'confessed' about their work for the security services. The RUC, in keeping with policy, refused to comment.
The IRA named the three as Aidan Starrs, John Dignam and Gregory Burns, well-known republicans who lived near each other in Portadown, Co Armagh, from where they had disappeared several days ago. Starrs, 29, was jailed for eight years for possessing explosives in 1983, and Dignam, 32, sentenced to 12 years in 1980 for causing an explosion, possession of a firearm and wounding.
Burns, 33, had no terrorist convictions, but his brother, Sean, was an IRA member shot dead by police near Lurgan, Co Armagh, in 1982. The IRA said their victim had supplied information to the security services on his brother.
The bodies, shot through the head, were discovered late on Wednesday night dumped within 10 miles of each other, beside isolated roads near Crossmaglen, Bessbrook and Newtown Hamilton.
All three had been killed after admitting working for the security services - Burns for MI5 and the others for the police, the IRA said.
It added that they were responsible for the 'brutal murder' of Margaret Perry, 26, Burns's girlfriend who disappeared from her home a year ago and was found buried in a shallow grave across the border in Co Sligo on Tuesday. Her body was only formally identified yesterday by her mother Mary.
Burns was recruited by MI5 in 1979 and paid to supply political information on Sinn Fein and to infiltrate the H Blocks Committees at the time, according to the IRA statement. His handler, code-named Frank, later sent him to live in Amsterdam to infiltrate the Irish community there.
On his return to the province in 1984, the Provisionals say he resumed his intelligence-gathering for which he was being paid pounds 220 a month. In 1987 Burns was urged to spy on the IRA's North Armagh brigade, through Aidan Starrs. The IRA say Burns was so well trained, both in accumulating details of weapons, and in creating innovative dumps for IRA weapons, that he ended up as quartermaster for the Portadown area.
By 1989, Starrs and Dignew, according to the IRA, were aware that Burns was working for MI5, and all three became involved in extortion and racketeering. The following year, the IRA claims, they were suspended while corruption allegations were investigated. The IRA contends that last year Burns feared that his girlfriend, Margaret Perry, might expose him and drew up a murder plan with Starrs, in which she was lured to Sligo and strangled and battered to death with a shovel. Starrs believed he had left too many clues, and, according to the IRA, returned with Dignam to the scene of the crime to retrieve taping he left behind.
Weeks later Dignam was arrested by the police and told of the murder but was given immunity for becoming an informant himself. Starrs was also arrested and confronted with Dignam's story, and he too was given immunity and recruited an informer.