Is it permissible for a young man to tell a girl about his love for her and his desire to marry her, then go to her family to propose to her and reserve her until he finishes his university studies, or should he propose to her now for three years, or what is the solution to avoid losing her?

If the girl is religious and well-mannered, then be keen to marry her; for the Prophet, peace and blessings be upon him, said: "So win the religious woman, your hands will be soiled (i.e., you will prosper)." A religious woman is more likely to know her husband's rights. There is no harm in expressing your desire to marry her, within the limits of Islamic law and without seclusion. If she agrees, then propose to her family and agree to postpone the marriage. If you strongly believe she will refuse, then do not propose to her and dismiss the idea. It is not permissible for you to tell her you love her, because she is a non-mahram to you, and such phrases can be a cause of temptation, and Allah the Almighty says: "And do not follow the footsteps of Satan. Indeed, he is to you a clear enemy."