I can see why he lied. Doesn't make it right, but I get it.
Some women want to be very involved with their bf's children to the point they want to join in all activities, birthdays, holidays and events. Even try to befriend the mother and cement themselves in the man's life through the child. If the man wants to keep things separate, the new gf can become offended, jealous and accuse the man of not loving her enough or not taking their relationship seriously if he won't "open up" all aspects of his life to her.
Not a great way to start a relationship and the temptation to avoid any potential drama where children are concerned, can be strong. When a woman doesn't allow a new love interest into her children's life she's a protective mother. It's not allows viewed the same with men.
I've seen this more times than I haven't.
Not saying the Scorpio was like this but in the beginning of a relationship you never know how a person will react. 3 years is crazy long though, dude messed up.