The south-western most of a group of three cairns located along a track on Mynydd Epynt (see NPRNs 305014 and 305015, a further possible cairn site (NPRN 90076) lies c.235m south-west of Cairn III.). A 1934 excavation of one of the cairns by Col. John Lloyd, Hon. Sec. of the Breconshire Society found a layer of ash and black soil, as well as kerb stones, but no central burial. This led Lloyd to conclude that the cairns had originally been waymarkers for drovers.

This grass-covered cairn is circular, measuring between approximately 18 and 21.6m in diameter and between approximately 1.8 and 2.1m high. A trench runs on the top of the cairn from the centre and another ditch is likely indicated by the growth of rushes. In the 1997 Inventory of Breconshire, the RCAHMW identified two peristalitic stones to the south, however in their 2004 report CPAT records that `only one stone is visible and it is on the east side of the cairn?. They also note that this is the only of the three cairns not to be damaged by the trackway to the northwest.

Brynley F. Roberts has suggested that the Tri Chrugiau may be the cairns referred to in Walter Map's De Nugis Curialium (Distinctio ii: xi. `De aparicionibus fantasticis?), in which King Brychan of Brycheiniog supposedly deposited the right hands, `virile members? (mentulas eorum), and right feet of the defeated army of Deheubarth, which had been led by Triunein Vagelauc. Although Walter Map writes that the cairns were named after the body parts buried therein, these names do not seem to have survived.

(Sources: NMR Site File, Breckonshire/Earthworks/SN94SW; Walter Map, De Nugis Curialium, ed. by James, revised by Brooke and Mynors (1983), pp. 149?55 (esp. pp. 152?5); RCAHMW, An Inventory of the Ancient Monuments in Brecknock (Brycheiniog): The Prehistoric and Roman Monuments, Part i (1997), p. 104; Roberts, `Melusina: Medieval Welsh and English Analogues?, in Boivin and MacCana (eds), Melusines Continentales et Insulaires (1999), pp. 281?95 (esp. pp. 290?91); CPAT Regional Historic Environment Record, 2004)
A.N. Coward, RCAHMW, 03.07.2018