It's a little difficult to say, as Ex3 is still in it's early stages and all of the splatbooks released so far have been focused on the mechanics of the game, not the setting. There are a few things I can think of off the top of my head though.
There will be new types and new factions of Exalted, though details are still sparse.
- Liminal Exalted: Attempts to bring back the dead. it failed (because that is one of the two hard rules in Exalted) but a strange force, known only as the Dark Mother, hijacked the body for it's own purpose. Basically zombies with an Aspect based on the intent behind their resurrections. They can increase their stats by using body parts possibly similar to how Alchemicals can swap out their builds. They require bonds to the living in order to function and can actually die by being rejected.
- Exigents: the Exalted of small gods, these are unique beings as the average god does not have the strength to survive the creation of more than one Exalt. They also need the Unconquered Sun's permission and a piece of his power to catalyze the process. There are hints of a black market for these pieces. Due to their nature basically everything about them will be homebrew, even when they get their own rulebook.
- Getimians: These beings were pulled from the dreams of the Primordials Oramus and Sacheverell. They lived in alternate timelines only to be pulled into creation where everything they know is wrong. There is going to be something funky about their mote pools (Flowing and Still as opposed to Personal and Peripheral). They have their own internal Looms of Fate that they can use to bind others to the roles they had in the timeline only that Getimian remembers. Rakan Thulio, a first age Sidereal, has pulled an army of these into creation and is waging a war against heaven.
There are also some changes to the 'classic' exalted types as well.
- There is no longer a set hard cap on how many of each type of Celestial Exalt there is. If your GM wants more or less than the usual 300 Solars/Lunars they are free to do so.
- Exaltations only seek "Excellence" in their host regardless of morality so the Solar: good, Abyssal: bad dynamic is no longer a given.
- Speaking of; Abyssals might actually be interesting to play now, Onyx Path seems to be committed to actually making the Abyssals the Exalted of the Dead instead of the Exalted of Slaughter. The Deathlords and Neverborn will have a much lighter touch this time around, giving Abyssals more agency.
- Infernals are also getting a huge rework. In Exalted: Essence they are anarchists, revolutionaries and avengers. They have a more standard Caste system but also a demonic form similar to a Lunar's war form. they spend and regain motes faster than usual and many of their charms explicitly raise their Anima level. how much of this will transfer to 3e remains to be seen.
- Lunars lore got a bit of restructuring, for example he breaking of the castes was a deliberate move on their part and there has been no mention of chimeras. In fact the Wyld doesn't feature in their lore nearly as heavily and many Lunar elders have had their stories changed to reflect that. The Silver Pact is more about mutual aid and cooperation than taking down the realm or building societies but individual lunar factions do still follow those themes.
- Dragon-bloods are much more spread out across the world, with clans and even entire civilizations unaffiliated with the Realm or Lookshy in every direction. Terrestrial society (at least in the Realm) has been updated to be Matrilineal. In fact many aspects of the game have been changed to be more inclusive.
There are massive geographic changes (compare and contrast), Creation is bigger and all of the coastlines have been redone. To the Southeast there is the Dreaming Sea with a new Dragonblood society revolving around the Sea's oneiromantic properties, A new large island in the Southwest that is sacred to both the Lunars and the Dragonbloods that has seen constant war. A bunch on new cities have been added and changes have been made to returning established locations but their relevance will depend on where your game is set.
There are some explicit changes to the early history of creation, the books go out of their way to avoid using the word 'Primordial', instead going with 'Enemies of the Gods' so presumably the original Exalted were pitted against more than just the Primordials. After their victory however the Exalted turned on each other, not much is known yet but out of this conflict some Solars and Lunars made a series of political marriages. This is the new origin of the Solar/Lunar Bond and as such it is no longer guaranteed that a given Solar or Lunar will have one.
There are a few things that will probably lead to larger lore changes once books more focused on setting are released.
Sorcery for example: it is much easier to attain, you can essentially make up your own sorcerous traditions but several published version exist. Powerful elementals can just make you a sorcerer in exchange for a favour for example. Sorcery no longer relies on the user's Essence at all so mortals don't even have to awaken their Essence to gain a mote pool. In addition it has a "Workings" system where you can make permanent changes to the world, be that giving yourself wings or creating a new river that flows uphill.
That's the only big stuff at the moment but so far the current state of the lore is that your GM will be have to make most of it up themselves or rely on previous editions. Each of the Kickstarters have given out 1e books that will be "of use" in 3e (Creatures of the Wyld + the Rest of the Creatures, Games of Divinity, Savage Seas, Scavenger Sons, Blood and Salt, Exalted: The Dragon-Blooded, Exalted: The Outcaste and all of the Caste and Aspect Books) as well as the 2e books 1000 Correct Actions and Glories of the Most High: Luna.
It's entirely possible that absolutely none of this will have any impact on your game. I may be able to answer more specific questions but that's all I can think of at the moment.
