Llanfyrnach 3 was first noted in 1937, and by 1951 was in its present position in use as a gatepost on the east side of the track, to the immediate east of the farm buildings at Rhyd-y-Gath Farm. By 1981 the wall had been extended across the gateway and the stone is now just south of the present gateway, set against the hedge. It is overgrown and covered in moss. 300m west of St Brynach's Church (NPRN 422419). It thought that the stone may mark the location of an early medieval cemetery site and/or chapel in the vicinity of the farm.

The stone is a rough, irregular felsite pillar, which tapers towards the top. Measurements are given as 157cm height x 46cm width x 38cm diameter. Face A is incised with a linear Latin ring-cross with a small, circular depression in each quadrant (comparable to Llanfyrnach 2) thought to date to the 7th to 9th century.

Sources include:
Edwards, N. 2007, Corpus of Early Medieval Inscribed Stones and Stone Sculpture in Wales

N Vousden, 4 January 2018