Cactus branch rotting(?)

Hi all.

I noticed that one of the "branches" of my cactus (one of them, the small one) seems to be attcked by the rot. It's getting weaker and squishy and seems to got darker brownish.

Generally, in my experience, the branch is beyond saving, but is there a way (and how) to try and save remaining three branches. They seem healthy and firm, so if I can get rid of the sick one, hopefully the cactus will recover.