There is a main rule in Hinduism with the idea that everybody is born, everybody lives and then dies. This was the case for humans. My question is: Why did Brahma, Vishnu and Shiva follow this rule? What would be the point of following this rule when they are all seeing, all powerful and all knowing. What is the significance of following this rule?
The Gods have their own rules, their own Laws. Such rules are not the ones we humans are required to follow.
This is mentioned in the Vedas as follows:
DevA devAnAm anu hi vratA guh.
The Gods move according to the law of workings of the Gods.
Rig Veda 3.7.7
The Commentary on this mantra:
Even the Gods observe a law and that law is the law of Truth. For the Gods are themselves born of Truth, grow in Truth and live in Truth. They are emanated from the vast Being of the Supreme to carry out its Truth-Will in the manifestation. They uphold and follow the workings of this Will, its law of effectuation. They are both the guardians and functionaries of this Law.
Adapted from Sri Aurobindo.
I think you are mixed up in the definition of omnipotent and omniscient. The main religious concept of whole vedic thought was rta which is a moral order or cosmic force which governs spiritual motion of human beings. Even Gods can not escape rta, violators are severely punished.
Consider a powerful rocket like Saturn V that made lunar exploration possible it would produce a thrust of > 100 KN. That said the rocket still has to follow laws of gravty. So being able to overcome gravity does not mean the law of gravity cease to exist. Same thing is hold true for Gods. As mentioned by Rickross that Gods follows rule of Gods. Rule of karma does not apply on them as that is applicable for people who born on earth. However killing someone (brahmin/bystander) is a sin even for Gods and they have to atone the sins