I have a boxer that is 3 months old he does real good about going to the door to go out to potty & pee but sometimes not every day or week just sometimes he just pee in the floor what can I do about this?

He is only 3 months old, and still learning. If he only sometimes goes outside or on his paper/potty to pee then he is doing incredibly well for his age. Just remember to praise him when he does go where he's supposed to go.