I agree with the theory that Tol's companions are The Three. There is further evidence to support this when you consider information gained from Archbishop Dilla, when question about "The Oppalan Frontier", the book from "... and the abyss gazed back". She says the book is about 4 adventurers, 3 of whom fit the description of The Three, who gathered the Nivurian Stones and ascended. However, she does not describe the 4th companion. It would make sense for this 4th companion to be Tol, and the book all but confirms that the Three were ascended mortals, as Archbishop Dilla has further dialogue about a priest who preached this very idea. This book would also explain how the 4 of them ascended, collecting the Nivurian Stones.