Is 'a user' he or she? or none?

A user is most definitely "she", because it is living person you are referring to.

1. Can you use HE for a girl?
2. Why is it a user not an user?
3. Can I use they instead of he or she?

Can you use HE for a girl?

Instead, grammarians have historically said that the pronoun he in all its forms should be used when the antecedent that the pronoun refers to could be either gender. About 30 years ago, some people began objecting to the use of he in this way, saying that it did not include women and was sexist.

Why is it a user not an user?

The choice of a or an is made based on pronunciation, not spelling. The only reason the word an exists is because a followed by another vowel is awkward to pronounce. Since user is pronounced /ˈjuːzə/, starting with a consonant "y" sound, the article a is appropriate, and an is not.

Can I use they instead of he or she?

Use they to indicate a nonbinary or gender-neutral subject (doer) of a verb (action) instead of he or she. For example: They cook an amazing lasagna or They have an important meeting at noon.