ive heard good things about hoplo cats as far as being active bottom feeders. otherwise, pictus cats and various types of synodontis fit your description and tank.
Is this going to be the only fish in the tank? If not, what tankmates? What is your water like?
I'd recommend pictus or synodontis petricola. Any catfish that grows larger than about 6-7" is too big for this tank IMO. I had a lone petricola for many years, that was in different setups I had, ranging from a mbuna tank to a fully planted tank. Was a great fish.
50g tank, depending on tank mates but the 3ft length is quite limited.
Maybe a lone gulper or blochii.
A cat I do have that is always out and quite relaxed is my platysirilus mucosus, get to about 7" I think, mine has been 5-6" since I got it a year ago lol.