What is the ruling of the Sharia on demanding a share from the sale of a freehold apartment after waiver, mutual consent, and spending a period in the old rental apartment?

Firstly: Your third brother did well to return the apartment to its owner, as jurists have stipulated the invalidity of selling what the seller does not own, and the obligation to return the price to the buyer.

Secondly: If someone enters into a transaction and stipulates an invalid condition, not knowing it is forbidden, then the condition becomes void, and the one who stipulated it has the right to annul the contract.

Accordingly, the division of the inheritance (the apartment) that was carried out and agreed upon, whereby the rented apartment was your third brother's share, is invalid because it was based on something you did not own. Since it became clear to your brother that he had no right to remain in the apartment, he returned it to its owner, thus nullifying the previous division. Based on this, his right to the apartment remains as it was, and thus each of you owes him one-third of the share he received.