Warii na comes from warui and the particle na. It's pretty informal, even a bit base. You can maybe get away with using it with really close friends, but I'd avoid it.
Sumimasen is a sort of light "sorry," and can be used for "excuse me," for instance if you bump into someone in a crowd or just want to get by, and can even be used in place of "thank you" if you feel like you have some fault in someone having to do something for you. It's mostly used for light offenses.
Gomen nasai is used for more serious offenses, but it has some overlap with sumimasen (or at least it feels that way to me).