I have a boy cat annd two girl cats. He is twice their size but cowers from them if they hiss at him or anything. They take his food all the time. How can I get him to stand up for himself?

Cats tend to be matriarchal. They are asserting their dominance over him. Let them.

As long as he's not getting seriously hurt, and the hissing is just for show, then there's nothing to worry about. If you're concerned he's not getting enough to eat, then feed him in a closed room, so they can't get to it.