Which goddess is Nefertem's mother in the earliest sources?

Sekhmet is his mother in the Memphite triad, which is the arrangement most often reproduced, but Bastet and Wadjet are each named in other local traditions. It is not clear whether one of the three is original and the others are later local substitutions, or whether the triad itself is a Memphite systematisation of a god who began without a fixed parentage.