Summary
The Original Three is a belief, typically held by the members of the relevant species, that Dwarves, Elves and Gnomes were the first three species created by
The Dia.
The Myth
Naturally, such a belief cannot be verified, but it is accepted as the creation story within their own cultures. The root of the belief is that Dwarves, Gnomes and Elves were created in the image of three aspects of The Dia. The Dwarves, created for their physicality, in the image of Toar. The Gnomes, created for their intellect and curiosity, in the image of Nona. The Elves, created for their affinity for magic, in the image of Ispilla.
Variations tie gods such as Dugarth, god of the underground to the Dwarves as well, and Cephione, goddess of nature to the Elves, for instance, but the true myth are those three aspects with those three Dia.
Of course, in reality, all species have strong, clever and magically adept individuals, but its these three traits that define these races. In the early days, it was said that Dwarves' responsibility was tending to the land and building civilisation. The Gnomes were in charge of thinking up ways to become more efficient, and to advance society. Meanwhile, the Elves were dedicated in a similar manner, but purely in the field of magic, finding better and stronger version of spells.
The Big Four
These three species, together with Humans, form what is colloquially dubbed 'The Big Four' in respect to the world's sentient species, since they are significantly the four most populous. Humans don't factor into The Original Three myth, as such. However, a related theory regarding the creation of Humans is that following The Primordial Uprising, the Dia realised their creations held too long lifespans, and as a result created Humans as a blend of the three (strength, intellect and magical affinity), but with vastly short lifespans in comparison. Variations of that belief have their creation further into the Age of Discovery, if they believe that none of The Original Three existed in the Age of the Primal either, and those these were created at the dawn of the Age of Darkness.
I like the way you have included variations on the myth to incorporate different values.