He was found in North Street at 07:10 GMT on Saturday remains in a hospital in a critical condition.
A 28-year-old man of no fixed address and a 43-year-old woman from Brighton have been arrested on suspicion of robbery and remain in custody.
A 30-year-old Brighton man was arrested on suspicion of grievous bodily harm and is also still in custody.
Any witnesses are asked to contact Sussex Police.
Three people have been arrested after a man was found unconscious after a suspected assault in Brighton.