In Neil Gaiman's Sandman, Titania of Faerie is the only Fae queen that we ever see, however the tripartite goddess is present throughout the story, especially in the climax: The Kindly Ones.

In the first arc of Sandman, Morpheus calls upon Hecate, a triple-form goddess with Maiden, Mother, and Crone aspects.  Later, in a flashback, Orpheus is in Hades to gain back his love.  To do so, he sings a song that makes all of the denizens of Hades cry, including the Furies (Erinyes), the same tripartite goddess, a slight for which they never forgive him and may even have called the Bacchae upon him to tear him apart.  Finally, as the main antagonist of the climax, they appear as The Kindly Ones (Eumenides), again a tripartite goddess, responsible for hounding those who spill family blood.  In the first and last forms, they are clearly Clotho, Lachesis, and Atropos, as they are called by name.  In The Kindly Ones, they are shown at the beginning of the story, Clotho helping to start a weaving, and Lachesis working on it.  At the end, when their business is done, Atropos is seen cutting it short, a metaphor for the life, and story, that they have just ended.

These names, Hecate, Erinyes, Eumenides/Kindly Ones, are used to speak of them when operating for different functions.  Perhaps these are the names that Mother Summer spoke of.
I would think that the Summer Lady is most likely Clotho, though.  It seems to track.