All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Whatever the father gives to his children during his life time is considered a gift, and it is desirable that he observes justice when gifting them, but that isn`t an obligation, whether it is a Hibah(Donation, grant), a charity or a gift. This is provided that no harm befalls any of the children as a result and that the father harbors no intention to deprive one of them from their share in the inheritance, because The Messenger of Allah said, "Fear Allah, and observe equity in case of your children." It is well known that observing justice when gifting children enhances the bond of love and mercy between the children(Beneficiaries) and their father(Giver), and between the children themselves as well. Therefore, the learned men of the faith have ruled that it is disliked for a father to favor a child over his other children as far as gifting is concerned, unless their needs varied or he did so for a sound Sharia reason.
In fact, you (Asker) could check on our website an article written by the Grand Mufti on this topic. In light of your above question, it is permissible to give, as gift, the children of the second wife an amount equal to that spent on the three daughters from the first wife and their education. It is also permissible to gift the second wife since the first wife is already divorced, and so she doesn`t take the rulings of a wife anymore. To be on the safe side as far as the teachings of Sharia are concerned, it is better to register the apartment in the name of the children from the second wife, but this isn`t obligatory. This is because spending on the three daughters was driven by a sound Sharia reason{Education}, and observing equity amongst children in such a thing isn`t an obligation. And Allah knows best.