Praise be to Allah, the Lord of the Worlds; may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
The apartments that your sons have built from their own money are theirs and you must register them in their names or write an affidavit to that end in a Sharia court so as not to be divided amongst the heirs after your passing away. As for the fourth flat, it should remain in your name.
As regards the rented apartment, if your son who had built it with his money didn`t object to your taking the rent then Allah Will Reward him for that and you are cleared from liability. However, if he wants the rent then he must be compensated for it unless you and their mother are poor and have no source of income, in which case all the children must provide for you and meet your basic needs. Moreover, the rent that the father took is considered part of the provisions for which that son is liable. In addition, the children may sit together to reach an agreement as far as spending on their parents is concerned. This is safer for everybody in terms of sin and injustice. And Allah The Almighty Knows Best.