From Archaeologia Cambrensis (1858), in an article about the history of Radnorshire:
Near to the above-mentioned place, Cwystudwen [Gwastedyn, Nantmel], are two remarkable carns, named Carnwen, and Carnfach, that is, the white and the little carn, each being of an elliptical form, and having in the centre an erect stone of superior magnitude ...
On the eastern extremity of this hill, and on a farm named Gifron, is a place which the common people distinguish by the appellation Gwar-y-beddau, that is, the ridge of graves; it consists of three mounds, or elevations, in which tradition reports three brothers, who, returning from the wars, quarrelled, fought, and fell by each others' swords, were interred.
In Coflein's description, Carn Wen is described as a much robbed cairn with a bouldered kerb and a possible cist. 'A battle-axe, a bracelet and some other relics' were found in 1844. The three cairns in the folklore are said to be 20-110m to the north east. Coflein calls the other cairn 'The Druid's Circle' - it might be a prehistoric enclosure with a roundhouse inside.
They are mentioned in Camden's Britannia (originally published 1607, this from the 1722 edition):
On the top of a hill, call'd Gwastedin, near Rhaiadr Gwy, there are three large heaps of stones, of that kind which are common upon mountains in most (if not all) the Counties of Wales; call'd in South-Wales Karneu, and in North-Wales Karned-heu. They consist of such [?] stones from a pound weight to a hundred, &c. as the neighbouring places afford; and are confusedly pil'd up without any farther trouble than the bringing them thither, and the throwing them in heaps.