Elves, orcs and humans become largely blurred together into a racial amalgam as the handful of survivors from each race are forced to turn to each other to expand their breeding pool. This is doubly true if all the survivors participate in a mass migration to roughly the same region.
Flash forward about 500 years. "Pureblood" members of one, two or even all three races may be looked upon as the new nobility - orcs for their strength, humans for their versatility, elves for their longevity. Alternately, purebloods may be looked down upon, with power residing in a virile young hybrid race and the rulers being those who claim descent from the kings and chieftains of all three races in the pre-cataclysmic age. A third possibility is that purebloods and certain stable half- or quarter-breed strains form a rough caste system - long-lived elves as lorekeepers, strong, smart orc-elves as the warrior-sorcerers, creative humans as the inventors and engineers, sturdy half-orcs as the manual laborers and so on.
Because of the strength of this triune amalgam, the other races, less commonly interbred with the "Big Three," will suffer much worse and probably not be able to establish significant communities in the next few centuries. Halflings and gnomes (and perhaps goblins) may form a small-sized mirror of the human-elf-orc hybridization, but neither of these races are known for amassing great power like the "Big Three." They are more likely to be scavengers of civilization than its driving force.
Dwarves, likely to be the only real purestrain race left, may organize themselves in opposition to mixed races. Being probably the best suited single race for this kind of environment, they could present a formidable foe. They might also choose to remain in whatever remains of the original, shattered continent.
Hobgoblins and bugbears don't seem to mingle readily with humans, elves and orcs. They are more likely to remain much as they are, nomadic raiders and barbarian hordes. Their natural enemies, the dwarves, now become natural allies in the face of the powerful "Big Three." They may serve as mercenaries, giving dwarves plausible deniability when they attack the "Big Three," or as a buffer between the two empires. Alternately, hobgoblins might forge an empire of their own, similar to the one described in Eberron's history.
Thus, we have an expansionist empire of the "Big Three," a parastic or sybiotic subculture of the "Little Three" or "Little Two," and the menacing presences of the defensively-minded but also bitter and hateful surviving dwarven empire. Furthermore, either large but disorganized goblinoid hordes and/or a hobgoblin empire with similar attitudes to the dwarves.
Class-wise, barbarians, rogues and rangers are dominant - scouts abound if the Complete Adventurer is in use. Most divine spellcasters outside the dwarves are druids - both elves and orcs have a strong druidic tradition - and most arcanists are sorcerers. Fighters also appear regularly. Paladins and true clerics are extremely rare, likewise wizards and bards.
