[gemy]

I don't want to burst anyone's bubble, but at three years old marking is firmly established, and neutering only has a small chance of changing that habit.

Belly bands with a pad insert frequently changed should be placed on the dog when he is allowed to roam free in the house. The use of an enzymatic cleaner is a must for any accidents

Urinating in the house is a linked but different issue. You should be able to housebreak that with due diligence.

And while not common some females actually do mark their territory as well.