The dating clause of a charter dated Apr 989, which records a donation by "Lando…episcopus sanct sedis Pestane", refers to the sixth year of "principatus domni nostri Iohanni…principis" and the first year of "principatus domni Guaimarii principis filio eius". A charter dated Oct 989 names "domne Sichelgaita principissa uxor…domni Iohanni…principis" and "domnus Guaimarius princeps et Iohannes comes germani filii…domni Iohanni et domne Sichelgaite". The Catalogus Principum Salerni record that "Weimarius princeps eius frater [=Wido] benignus et clemens" succeeded as Prince of Salerno and ruled for 43 years 9 months.
He succeeded his father in 999 as GUAIMAR III Prince of Salerno. The dating clause of a charter dated Oct 999, under which "Alfanus filius quondam Petri de Capazzana" donated property to Cava monastery, refers to the eleventh year of "principatus domni nostri Guaimari…principis" without naming his father. Normans returning from a pilgrimage to Jerusalem helped him defend Salerno from Muslim invaders [1000] and were invited to stay, the first Normans to settle in southern Italy. [The dating clause of a charter dated Sep 1006 refers to the eighth year of "Guaimarii filii quondam Guaimarii principis". It is assumed that this charter is misdated and should refer to Prince Guaimar IV as no other indication has been found that the father of Prince Guaimar III was named Guaimar.] Prince of Capua and Duke of Amalfi [1010]. Duke of Sorrento [1011]. The dating clause of a charter dated May 1012 refers to the twenty-fourth year of "principatus Salerni domni…Guaimari…princeps", the fourth year of "principatus eius Capue et ducatui Amalfi", the third year of "ducatus illius Sirrenti", and the first year of "suprascriptorum principatuum et ducatuum domni Gisulfi…principis filii eius". The dating clause of a charter dated Nov 1018 refers to the thirtieth year of "principatus domni…Guaimari" and the first year of "principatus domni Guaimari eius filii…principibus".
The date of his death is estimated from his being named with his son for the last time in the dating clause of a charter dated Feb 1027, and the dating clause of a charter dated Jun 1027 referring to the ninth year of "principatus domni…Guaimari" and the first year of "principatus domne Gaytelgrime genetrice eius". The Annals of Romoald record the death in 1030 of "Iohannes princeps Salerni" and the succession of his son "Guaimarius", but the charter quoted above shows that this date must be inaccurate. 1
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