The three alchemical principles were two for several centuries before they were three, and Sulphur is one of the original pair.
The two-principle theory
The doctrine that all metals are composed of two principles, a sulphur and a mercury, comes out of the Arabic alchemical tradition and passed into Latin Europe with the translations of the twelfth and thirteenth centuries. It is a theory of metals, and it did real explanatory work: differences between metals were accounted for by the proportions and the purity of the two constituents, which made transmutation a question of adjusting a mixture rather than of creating a substance from nothing.
Not the yellow powder
The principle called Sulphur is not brimstone. Alchemical writers were explicit that they meant a philosophical sulphur, a principle of combustibility and of fixed, formative nature, and that the mineral of the same name was at best an instance of it. Readers who take the principle for the element will misread almost every passage in which it appears.
How a third arrived
A third principle, Salt, was added in the sixteenth century in the work of Paracelsus, giving the set of three, the tria prima, that this atlas records. The addition is datable and attributable, which is unusual in this literature, and it made the scheme a theory of all bodies rather than of metals alone.