and then appear in almost every volume, in one aspect or another, until finally they appear as the Kindly Ones and
bring ruin to the Dreaming and force Morpheus to give up his existence.
Morpheus describes them as obeying the oldest rules, in the same way he does. The Three-in-One appear to be a fundamental force in the Universe.
And then there's this panel after the events described above - to me, it appeared as Neil Gaiman's monologue, reflecting on the plot of The Sandman.
It is never explained what exactly they are, which brings me to the question - what do the three sisters symbolise? The inevitability of fate? Some sort of higher power? Determinism? Taking the responsibility for own actions?