Gregory III was the son of Gregory II

Gregory II, Count of Tusculum

Gregory II was the son of Alberic III, Count of Tusculum and Ermelina. He was the Count of Tusculum and the Lateran from 1044 to his death....

. He was the Count of Tusculum from 1058, when his father died. He is usually considered to stand at the fore of the family of the Colonna.

Unlike his father, he did not bear the ducal and senatorial titles, nor any title to Rome, but simply was patricius et consul, (Comes Tusculanensis, Consul, illustris). Gregory was succeeded by his eldest son, Ptolemy I

Ptolemy I of Tusculum

Ptolemy I was the count of Tusculum in the first quarter of the twelfth century. He was a son of Gregory III...

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He had sons named:

• Ptolemy I (1099–1129)-(Consul comes Tusculanus);
• Gregory IV (1109–1128)-(Iudex Tusculanensis);
• Peter de Columpna (1099–1151) - (Colonna family
  Colonna family
  The Colonna family is an Italian noble family; it was powerful in medieval and Renaissance Rome, supplying one Pope and many other Church and political leaders...
  
  );
• Egidius (1128–1137);
• Iadara (daughter).

Sources

• Foundation of Medieval Genealogy: Northern Italy — 21. Counts of Tusculum.