huh. Three? Do you go with nations? Religions? Deities? Species? Geological/-graphic regions? Cultures? Languages? Classes? Organziations? ...or...or...or...
Well, it's my setting, so I think ALL of it's cool. hahaha. This is hard....So, hmmm...I'll try to go in different directions.
1. Location: The "Lost Sands," a.k.a. the Thelitian Desert, a.k.a. "The Seven Sands," a.k.a. the Empire of Thel. A region of magical "fallout" from the calamity of the Godswar. Currently the expansive deserts (seven different desert regions, in all) that constitutes the Empire of Thel ("Thelitia" to those outside the deserts). Magic there is chaotic. "Wild magic," in the 5e vernacular, for anyone not native to those realms. Only those born and raised (taught and trained) in the desert environments are capable of reliable, "proper," spellcasting. This makes the mages of the indigenous Thellic humans extraordinarily powerful within Thel. The other species of the sands: a felinoid people of the border grasslands, and salamanders who inhabit/"rule" (insofar as anyone can rule anything there) the Flame Desert, are also able to cast normally. Outsider mages are at a severe disadvantage within the realms of Thel.
2. Biology: True dragons are in decline. They have more and more difficulty producing wyrm/"true" dragon offspring. More often than not, the majority of a clutch of eggs (if not the entire clutch, in recent years), viable offspring are mutated 'draconic" creatures: hydrae (four limbs, multi-heads, possible breath weapon), behirs (serpentine, many limbs, no wings, breath weapons), linnorms (serpentine, two clawed forelimbs, no wings, venomous bite), wyverns (two rear limbs, wings, venomous tail, no breath weapon), dragonnes, and such like. So, that is where those kinds of creatures come from...and they are increasing. The number of actual "D&D dragons" is slimmer and slimmer. Gemstone dragons, specifically, are singular individuals. True wyrms are dismayed and at a loss for a remedy.
3. Organization: maybe not "the coolest" one, there are an awful lot of organizations across my setting. But, a particular favorite of mine, is the Riv Chaliir ("rihv sha-leer"). Translated from [my world's] elvish as the "Leaf Dancers." A mystic order of my world's largest "high elf" kingdom/realm. The Riv Chaliir are sacred archer-monks who are trained to the perfection of their arts for the express purpose of being assigned to defend the high elves' premiere council of sorcerers (the most powerful mages among the high elfin folk, tasked with the caretaking and protection of the magic of the world).