She probably already knows. For what you have stated, she seems pretty sure that she doesn't fancy boys.
I know she's only 3 years old, but that doesn't mean she's confused!
I'm 20, lesbian, and I always knew it.
Besides, what's really "normal" nowadays? If you always knew you liked boys, she can also know already she likes girls. It is the same thing.
Regarding other people's opinions, she will always have to face difficulties in her life (whether she happens to be gay in the end or not). But nowadays people is accepting more than you think.