Are the siblings entitled to claim their share of land that their father cultivated for three years, and then one of the sons took advantage of it, dug into it, and built upon it for twenty years with the father's knowledge and without his objection, knowing that the land was not owned by anyone?

If the father cultivates barren land in a way that constitutes revitalization, then he owns it, based on the Prophet's (peace be upon him) saying: "Whoever revives dead land, it belongs to him." What you have done by plowing, grazing, and building with his knowledge does not remove this part from the father's ownership, because it is a loan and does not count as transferring ownership. Or, if it is a gift, it is not valid unless the remaining children agree to it, in order to maintain fairness among them.

Therefore, this portion is included in the father's inheritance and must be divided among the heirs. You are entitled to the value of what you have developed on the land.

However, if the father did not revive this land, then he did not own it. Consequently, the portion where you built the house is yours due to your revitalization of it, and it is not included in the father's inheritance.