These are not spoilers but speculation, as I have not seen the raws or anything. I think the three hands are:
Suit sleeve = Yijun (reasons are obvious for this one)
Bandaged arm = Gyeongsu (I think it's his because he's in the hospital in a coma because of a suicide attempt)
Tattooed Arm = I think it's the bandmate he was talking to in the alley(I can't remember his name. There are no scenes showing his arms, but considering the MC is tattooed, it would make sense that his bandmate is, too. I'm basing this on nothing really besides a hunch)
I think when MC's bandmate finds out he's sleeping with a guy, he will make moves on MC. I think he'll confess that he has always liked MC but didn't think MC would be interested cause he's not gay.
I think that due to the money that Yijun is paying for the hospital, Gyeongsu will get better medical care and wake up. I think when that happens, he, too, will admit he's in love with MC and that his suicide was a result of losing his belief in his musical dream and hopelessness because of his unrequited love for MC.
Okay so it's 3 MLs? Is this poly? Is there already a main ML? PLEASE I HATE LOVE TRIANGLESSS