1) She must have been there yesterday. (I´ve come to this conclusion)
2) She must be there now
3) She must be there tomorrow (next week)
Is sentence #3 right? Can I ever infer about the future?
Just like svartnik points this out in his post, sentence #3 can't refer to a future event. The sentence as it stands merely says that it is necessary for her to be there tomorrow, or it is her obligation to be there tomorrow.
[more probable] She can be there tomorrow. (this use actually denotes possibility) She may be there tomorrow.
She might be there tomorrow.
She could be there tomorrow.
This sounds rather artificial to me. The last three all mean the same thing. If you want to express a probability, you'd use adverbs, not a different modal.
Would you actually make these distinctions in real life, engee?
* "She could be there ... actually, no, I'll go so far as to say she may be there tomorrow".
Much, much more likely:
"She could be there ... actually, no, I'll go so far as to say she probably will be there tomorrow".