It’s not about making it to our 30s to “get our acts together.” It’s that we have to make it to our 30s to get anywhere near the pay and benefits our parents and above got with a high school degree… if we’re lucky.
Your kid has a girlfriend. Yes, he probably right now cares more about you than her. Or at least has a stronger desire to please her than you. It happens.
You and the gf shouldn’t be talking about what kiddo gives her. If you want to make him pay, talk to him. There’s nothing you can legally do (except maybe sue where you’ll get nowhere). You just have to make sure it’s worth risking your relationship with him over.
3 years is 100% unreasonable.