Barjinder Singh Sangha, 33, pleaded guilty to the charge of "unlawful wounding with intent to do grievous bodily harm" at the hearing in Southwark Crown Court.
Mandeep Singh Sandhu, 34, and Dilbagh Singh, 36, pleaded not guilty to the same charge.
78-year-old Brar was attacked on September 30, 2012, in central London, while he was on a private visit here and was headed back to his hotel with wife Meena, who escaped unharmed and was instrumental in alerting passers-by with her screams to call the police.
Sangha also pleaded not guilty to a second charge of assault on Meena Brar.
"You are remanded in custody until April 2, when all three or two of you will appear for a trial. Mr Sangha, you will be sentenced for what you have done at the conclusion of the trial and not before," Judge Alistair McCreath told the three accused, who appeared in the dock wearing black turbans.
All three Sikhs sported long beards and spoke only to confirm their identities and register their pleas.
While Sandhu admits to being present at the scene but not having participated in the attack, Dilbagh Singh has refused to confirm or deny even his presence on the day.
The judge, describing the incident as a "serious attack", stressed that this lack of a definitive answer will be construed as a decision not to cooperate with the court.
Brar had sustained serious, but not life-threatening, injuries when suspected Khalistan sympathisers stabbed him in the neck and cheek.
The retired general, who spearheaded the military raid against Sikh separatists at Golden Temple back in 1984, was not present at today's hearing.
The court was told that he will give evidence in the trial via live video link from India.
