I don't think there was three. I think Jim put his own spin. There are 3 Fates. Mother, Queen, Lady. 3 Fates for Winter, 3 Fates for Summer. If there wasa 3rd Mother, what would she represent? Not a season as the other 2 do. A 3rd Mother would be out of place from the readers perspective. There is a precise balance between Winter and Summer, and a 3rd Mother wouldn't make sense.
EDIT: Unless before they took up Winter, and Summer, there were 3 Mothers, and the 3rd Mother leads the Fomor who Jim did say were defeated by the Sidhe.
life, fate, all things to come, there's a reason they had to throw her @$$ out when free will came about. Lachesis the spinner, decided fate for everything, in some stories even the Gods Couldn't defy fate.
Lachesis IS in DF if only apparent by her noticable absence. Her spin wheel actually made an appearance in SK, something I chalk up to one of those object/mantles MW seems to collect as trophies for particular kills.
As for a third not making sense, of course not, but as you say, they weren't originally formed as they are.. they created the courts later, those are not the original forms of them, least not the mother's who would actually matter on the right scale. Summer and Winter are smaller aspects of themselves they broke off.
Or on another wavelength, Norse mythology is fire and ice consuming the world makes a third power, Water. Kinda fomory that.
Slightly off topic tin foil theory
the missing fate and her connection to Nemesis (the goddess) I think are key elements in the DF. They tore her apart and broke down the different aspects and gave them out in different places. One piece of which I think, is wizardy itself. Magical the vessel for which Lachesis enforced fate would make alot of sense compared to the mythology. Wizards only get in trouble from violation of free will, the murphyonic effect is a basic byproduct of lady luck, alot of the bad guys are trying to recreate that image of a powerful young female wizard whose slightly unbalanced, ect.