Prabjot Singh Rayat, 54, owner of Rayat Autos, in east London, was found dead on 8 June on a small industrial estate where the garage is located.
A post-mortem at the Greenwich Mortuary the next day gave the cause of Rayat's death as a stab wound, the police said.
A 33-year-old man and a 44-year-old woman were taken into custody at south London police stations.
The police today said the third man, aged 39, was arrested in Gravesend yesterday on suspicion of murder.
The first two arrested appeared in court today to confirm their names.
The police did not name them, but reports identified them as Robert Burrows, 33, and Surinder Chana, 44.
