쓸모없다 and 쓸데없다 are close in meaning but their use cases are different. 사용없다 is unnatural expression you never hear or even see in a dictionary. * 쓸모없다 = useless (It's typically used about practical application). * 쓸데없다 = serves no good purpose (Has a broader usage, like judging what people say or do). So you say 쓸모없다 about some physical thing that you don't see any use for. E.g. 이 세탁기는 건조 옵션이 있는데 별로 쓸모가 없어 = This washer has a "dry" option but it is not very useful. 쓸데없다 can describe anything that doesn't help with a situation. E.g. 너는 왜 쓸데없는 짓을 하니? = Why do you do such an uncalled-for thing. 쓸데없는 말 (하지)말고 잠이나 자 = Just go to bed instead of talking such nonsense.