Graeme Jarrett Vickerstaff, 47, pleaded guilty on Wednesday to one charge of manslaughter, joining Dean Phillip Welsh, 50, and Colin Maxwell Booth, 35, who previously pleaded guilty to manslaughter.

Court documents previously tendered stated the three men had gone to Allan's Goodenia Street home in the early morning and attacked him when he came to the door, before robbing the home of drugs, a phone and bank cards.

Mr Allan was a drug dealer and the men believed he was in possession of a large amount of cash or drugs. Vickerstaff also believed Mr Allan may have been intimately involved with his girlfriend, the documents suggested.

Vickerstaff, accompanied by the other two men, was armed with a baseball bat and sprayed Mr Allan in the face with degreaser before beating him with the bat.

Welsh contended he had gone to the home to get drugs from Mr Allan and then either opened the door or held it open while Vickerstaff went inside and beat Mr Allan with the bat.

Booth said he stayed outside and only went inside after the attack, to steal the drugs and other items.

Severely injured and bleeding, Mr Allan dragged himself down the street to a neighbour's house, where he knocked loudly and yelled out "Please".

The terrified occupants called the police and when they arrived they began CPR on an unconscious Mr Allan. There was a 100 metre trail of blood down the street.

The documents said Mr Allan died of cardiac arrest, "which in turn is a consequence of the combined effects of blunt force head trauma, exertion, cardiovascular disease, and methamphetamine toxicity".

Welsh and Booth left Rivett in a taxi and stopped at McDonald's in Manuka, where CCTV footage captured them in clothing stained red.

Later, they used Mr Allan's bank cards to buy alcohol.

The three guilty pleas mean a lengthy ACT Supreme Court murder trial was avoided and the three men will appear in court in December for sentence.