He was the son of Count Reinhard II of Hanau and his wife, Catherine of Nassau-Beilstein.

In 1434, when his father was still alive, he took over the guardianship of the children of his widowed sister from her first marriage with Count Thomas II of Rieneck, when she remarried with Count William II of Henneberg-Schleusingen.

A Late Gothic winged altarpiece at Wörth am Main from around 1485-1490 – originally from St. Mary's Church in Hanau – depicts Count Philip the Younger and his ancestors, including Reinhard III and his wife.

Since this image was painted 40 years after Reinhard's death, it is reasonable to assume that it is not very accurate.

He married on 11 July 1446 with Countess Palatine Margaret of Mosbach (2 March 1432 – 14 September 1457).