What are the consequences for a man who does not know whether he entered into a state of ihram for an uncompleted Umrah three years before his marriage, and recently learned that his marriage contract might be invalid if he had entered into ihram and not exited from it?

The intention for Umrah is not considered an ihram (ritual consecration) that prohibits marriage. The default state is not being in ihram unless a man is certain that he has entered ihram. Doubt does not alter certainty, and the ruling of certainty is upheld while the ruling of doubt is discarded.