Timaeus
The Receptacle (52d)
Timaeus: both one and two. Let this, then, be, according to my verdict, a reasoned account of the matter summarily stated,—that Being and Place and Becoming were existing, three distinct things, even before the Heaven came into existence; and that the Nurse of Becoming, being liquefied and ignified and receiving also the forms of earth and of air, and submitting to all the other affections which accompany these,
The Three Initial Hypostases (8)
This is the explanation of Plato's Triplicity, in the passage where he names as the Primals the Beings gathered about the King of All, and...
(8) This is the explanation of Plato's Triplicity, in the passage where he names as the Primals the Beings gathered about the King of All, and establishes a Secondary containing the Secondaries, and a Third containing the Tertiaries.
He teaches, also, that there is an author of the Cause, that is of the Intellectual-Principle, which to him is the Creator who made the Soul, as he tells us, in the famous mixing bowl. This author of the causing principle, of the divine mind, is to him the Good, that which transcends the Intellectual-Principle and transcends Being: often too he uses the term "The Idea" to indicate Being and the Divine Mind. Thus Plato knows the order of generation- from the Good, the Intellectual-Principle; from the Intellectual-Principle, the Soul. These teachings are, therefore, no novelties, no inventions of today, but long since stated, if not stressed; our doctrine here is the explanation of an earlier and can show the antiquity of these opinions on the testimony of Plato himself.
Earlier, Parmenides made some approach to the doctrine in identifying Being with Intellectual-Principle while separating Real Being from the realm of sense.
"Knowing and Being are one thing he says, and this unity is to him motionless in spite of the intellection he attributes to it: to preserve its unchanging identity he excludes all bodily movement from it; and he compares it to a huge sphere in that it holds and envelops all existence and that its intellection is not an outgoing act but internal. Still, with all his affirmation of unity, his own writings lay him open to the reproach that his unity turns out to be a multiplicity.
The Platonic Parmenides is more exact; the distinction is made between the Primal One, a strictly pure Unity, and a secondary One which is a One-Many and a third which is a One-and-many; thus he too is in accordance with our thesis of the Three Kinds.