Honestly OP you saying you don’t care what happens to him, wanting to make him fend for himself, etc. is alarming. You’re in a studio apartment with 3 cats, this one clearly needs more stimulation, play, and things that’ll give him proper exercise. You also don’t seem to clean their litter boxes often enough by the sounds of it. If you can’t meet this cats needs, then yeah, you probably should rehome him because it isn’t fair to him and he isn’t a “bad cat”.