YTA.

Yes, grown adults do often get engaged after three years together. It does not take somebody in their mid twenties or older three years to decide whether or not they want to marry somebody.

It’s one thing to want to marry her but need more time, but that’s not what happened here. You admit yourself that you don’t want to marry. You know that she does. So yes, you strung her along knowing marriage was important to her and that you were never going to give her that. When she asked you about marriage and kids, you vaguely said that you weren’t ready without giving any detail or without saying that you only meant for kids and that you never intended on marriage, because you knew if you expressly said “I don’t want marriage, but I do want kids - I’m just not ready yet,” she would’ve left. You lied to keep her with you. You don’t get to feel betrayed after you wasted three years of her life.