Is it permissible to divide a gift equally among children after one-third of a house was gifted to the eldest daughter, and then the father was blessed with two more children, knowing that the competent authorities refuse to register the new gift before the daughter reaches the age of majority?

It is not a condition for the validity of a gift that it be registered with official departments. Thus, you can attest to the gift of one-third of the house to all your children, mentioning your revocation of a previous gift to your daughter and making the aforementioned one-third equally distributed among them. It is preferable to legally register this to safeguard rights. However, for the validity of a gift of a residential house, it is a condition that it be vacated and that the donor does not return to it before one year has passed. A gift is only valid through possession (حوز), and for the validity of a gift of a residential house to a child, the father's possession of it and attestation are not sufficient. Rather, the donor must leave it and remove their furniture from it.