Mercury is the other half of the older pair, and the ambiguity in its name caused more confusion in the literature than any other single term.
The two-principle theory
The doctrine that metals are compounded of a sulphur and a mercury reached Latin Europe from the Arabic alchemical tradition through the translations of the twelfth and thirteenth centuries. Within that theory Mercury supplies fluidity, volatility and the metallic character, and Sulphur supplies combustibility and form. Adjusting the proportion of the two accounted for the differences between metals, which is what made the transmutational programme look tractable.
Three different things under one word
Alchemical texts use the word for the philosophical principle, for quicksilver the actual liquid metal, and for a prepared substance the author claims to have made and will not name. Writers were aware of the ambiguity and some exploited it deliberately, since a treatise that could be understood by an adept and not by anyone else was doing what its author intended.
That deliberate obscurity is a genuine feature of the corpus and not a failure of the modern reader.
How a third arrived
Salt was added as a third principle in the sixteenth century by Paracelsus, producing the tria prima. That addition is datable and attributable, and it extended a theory of metals into a theory of all bodies.