Your first two sentences are incorrect. All you have to do is be confident. Correct. All you have to do is to be confident. Incorrect.

i think 'All you have to be is confident' makes the most sense. when saying it, it sounds better and flows better :)

"All you have to do is be confident," makes the most sense. However, if some one asked you, "what do I need to be?", then "all you have to be is confident." is acceptable as well. Just know that to say "all you have to be is confident" is what is considered a "turn-of-phrase," where the person asking you is likely asking for a specific role or office they need to serve and, instead of giving them exactly what they ask for, you are simply telling them not to concern themselves with what they will do but to simply act confidently when they do it. A person who says this would do so hoping to cheer the person asking and lighten their mood.

I think the right one looks like this: "All you have to do is to be confident!"

Sorry I was trying to help but I wasn't correct.