It unlikely, the way I see it (unless Jim is severely rewriting mythology, or just got it very wrong) that anyone but Mother Winter could be either Skuld or Atropos. Which I understand the issue, because the youngest of the triple goddess should be Skuld (i.e one of the Ladies) but clearly the mantle fits better in Mother Winter's Death Aspect.
Here is my interpretation. We see the Queens as separate - because we view the world through Harry's limited human perception. However the "truth" could be that ALL of the Winter Queens are Winter (of which Mother Winter is the least human) and ALL Queens of Faerie total up to something greater...there is every chance that Mother Summer's use of "our" included ALL Queens of Faerie. The Name of such a being might well be Fate, Gaia, God or something else that Jim has dug up/created. I think this is why Harry is partly wrong/stupid because he only see's the Mother's linked, not all the Queens adding up to something greater than the some of their parts.
Their are many, many versions of the Triumverate in many religions and mythologies. Triple Goddess of Wicca, Brahmic Trinity, Devi Trinity, Catholic Trinity, Greek Fates, Norse Norns, Egyptian Ra and many others. I have long wondered if sum of the Queens represents all of that.