Is it permissible to marry without the knowledge of one's parents or a formal contract to avoid what is forbidden, given their insistence on delaying the marriage for three years on the pretext of studying?

The only legitimate way for a young man and a young woman to have a relationship is through a legitimate marriage, which is not valid without the presence of a guardian and witnesses. If the young man is not able to marry now, he must fear Allah and practice self-restraint. He can be aided in this by frequent fasting and guarding his hearing and sight.