Children develop at different rates.
If you do hear your child do this, then correct her. If she is not corrected, she will carry on doing it and won't know that this is not the correct way to refer to herself.
Only correct her at the time she speaks it the wrong way. You will find that it can take quite some time to correct, especially if she has been doing it for a while.
Does her teacher or someone around her do it?
I'm really surprised how often adults speak in the third party to children. "Can Michael throw me the ball?" "Let Daddy put a bandaid on that".
She probably heard it somewhere. If you don't like it, don't react to it or mention it. It then may go away. good luck