John III (died 1008 or 1009) was the consul and duke of Gaeta from some time between October 984 and January 986 until his death.
He was the eldest son of Marinus II,[1] who succeeded his brother Gregory in 978 and immediately appointed John as co-duke in order to assure his inheritance, as the precedent of fraternal inheritance had been set by the sons of Docibilis II.
Also in 992, John's brothers Gregory and Daufer appeared as counts in Castro d'Argento and Traetto.
In 999, Emperor Otto III confirmed the independence of the various appanages of Gaeta (Fondi, Traetto, etc.).
Despite this great reduction to direct Gaetan power, his brothers remained faithful to John and even treated him as the first among equals.