In general you will only say "never ever", unless you are extremely sure about it in which case you'd say "never ever ever", but that is only in spoken English. If written the double ever is redundant.

Just "never ever" is usually enough. If you want to add more words, then add "ever": "never ever ever..." We already have the negative (never), so no need to add it again.

Have you ever heard this song ? :P http://www.youtube.com/watch?v=WlisjA9WDJU

never say never.......