Why would a ex narcissist boyfriend that broke off a three year relationship with me begin showing up at my child's sports activities she is not his daughter she is from my previous marriage?

He still has an obvious interest in you and it sounds like it is on the border of becoming overbearing and stalking. If this behaviour continues and you do not want any contact from this person you may have to report this - he could become very controlling if he doesn't get what he wants as a narcissist. Also if you do not want him around your children you should have a talk with them to tell them not to talk to him or go near him - especially showing up at her soccer game. Take care & be careful.