The exact identity of Hermes Trismegistus is uncertain, with at least three possible interpretations popular among the Order. One is that this is the Hermes, the Greek god, patron of travelers, wizards and thieves. The second possibility is that this Hermes was a mortal man, or even a dynasty of mortal men--perhaps the "thrice-great" title is meant to refer to three generations of the same family? Some supporters of this theory identify Hermes with Pythagoras, the Greek philosopher who was known to have studied among Thoth cults in Egypt, although the Corpus Hermeticum was apparently written some centuries after his death.  The final theory is that "Hermes" is a meant allegorically; Hermes is not actually a god or a man, but an archetype representing communication, discovery and wisdom.
For many in the Order, Hermes is all three of these at once, and the exact details of the Corpus' author are far less relevant than the metaphysical insight.