[1] Frederick was the son of Count Dietrich IV of Moers and his wife Elisabeth of Zuilen, heiress of Baër.
[2] On 10 September 1376, Frederick married Walburga (sometimes spelled Walburgis), the daughter of John II of Saarwerden and his wife Clara of Vinstingen.
The marriage had been mediated by Walburga's brother Frederick, who was Archbishop of Cologne and wanted closer a relationship with the Counts of Moers, to reduce the influence in the Lower Rhine area of his nemesis Adolf III of the Marck.
In 1397, Walburga's brother Henry II died without a male heir, and Frederick III inherited the County of Saarwerden jure uxoris.
This meant that if a Count were to die without a male heir, the county would be inherited by a younger brother (or his son), and not fell back to the Duke of Cleves.