(a) If notice of a hearing on any petition is required and, except for specific notice requirements as otherwise provided, the petitioner shall cause notice of the time and place of hearing of any petition to be given to any interested person or his attorney if he has appeared by attorney or requested that notice be sent to his attorney. Notice shall be given:

1. (1) by mailing a copy thereof at least twenty days before the time set for the hearing by certified, registered, or ordinary first class mail addressed to the person being notified at the post office address given in his demand for notice, if any, or at his office or place of residence, if known;
2. (2) by delivering a copy thereof to the person being notified personally at least twenty days before the time set for the hearing; or
3. (3) if the address or identity of any person is not known and cannot be ascertained with reasonable diligence by publishing a copy thereof in the same manner as required by law in the case of the publication of a summons for an absent defendant in the court of common pleas.