Tripod

Three mentions:

The inspiring goddess entered a mortal’s breast,
Made there her study of divining thought
And sanctuary of prophetic speech
And sat upon the tripod seat of mind:
     — Book 1 Canto 3

Thought sat, a priestess of Perversity,
On her black tripod of the triune Snake
Reading by opposite signs the eternal script,
     — Book 2 Canto 8

He has lost the inner Voice that led his thoughts,
And masking the oracular tripod seat
A specious Idol fills the marvel shrine.
     — Book 4 Canto 3

Tripod is a three-legged stool that served as the seat of the Pythia in the shrine of Apollo at Delphi.

Tripod Tripod

Dispute of Heracles and Apollo for the Delphic tripod. Louvre Museum. Public domain

This tripod could represent the three lower planes of the human mind – physical, vital and intellectual – as in the line “And sat upon the tripod seat of mind”.

But it is more likely to symbolise the three modalities of reception of higher truths – Intuition, Revelation, Inspiration – which characterised the oracle of Apollo in its early days, before they became obscured over the centuries (See Julian Jaynes, The Origin of Consciousness in the Breakdown of the Bicameral Mind).