[matese]

Welcome to YT.
Even if you neutered him now, at the age of 3 would not stop the marking.

I have a boy soon to be 8 years old, he was neutered at 8 mos old, he is outside trained on, he uses no pee pads, he has never marked in the house. 95% of the time he squats when he pees, he only hikes his leg if we go someplace he has never been to.
I suggest start the re-training you did when he was a puppy, confine him to one area where his pad is when you leave him alone or when you are home and unable to keep an eye on him. Night time will be hard, if he is crate trained I would keep him in the crate.

Hopefully other members with male dogs will pop in and give other suggestions.