Is refraining from paying back one-third of the father's debt, from which one of the siblings benefited to the exclusion of the others, considered disobedience to parents (Uquq al-Walidain) in Islam, and what is the Islamic legal ruling on this matter?

You are not legally obligated to pay off the debts of the two apartments your father gifted to your brothers. It is not just for a father to prefer some of his children over others in gifting. Nevertheless, it is advisable to obey your father and pay some money towards the debt as a form of filial piety. If you are unable to do so, apologize in a polite manner.