He is traditionally believed to be the second ruler of Moldavia bearing this regnal name, though some historians (based on a Polish chronicle) have posited another Petru had ruled Moldavia in the late 14th century, making him the third Petru(Peter) in regnal order.
He came to power to his first and last rules with help from Hungary's Governor General John Hunyadi.
According to Polish chronicler Jan Duglosz, Petru married in January 1448, when he was still 25 years of age, Hunyadi's older sister (name unknown) who was 50 years old at that time.
During his reign, a 3000 men cavalry contingent supported John Hunyadi's 1448 campaign, that ended with the second Battle of Kosovo (1448).
Petru III also ceded fortress of Chilia/Kilija (nowadays Chilia Veche) to John Hunyadi.