En av utvecklarna av 3: e upplagan svarade på samma fråga på den här rpg.net-tråden så här:
Avatars/archetypes and adepts/magickal schools work the same. There's a slightly different focus on different ones in the new edition, though you can easily port the old ones over. UA3 throws out attributes and skills and rebuilds all characters around the shock gauge, which is the renamed madness meters. Your hardened notches translate directly into core abilities like Dodge, Fitness, Knowledge, and Notice, but you can substitute your identities (%-based "skills" that encompass what your character mostly strongly identifies with) in many cases.
It still uses % dice, you still have flip-flops and matched success/failures and other things. In general however it is much more oriented toward "what do you actually do in this game?" than before. Character creation is a group session involving drawing up a relationship map and coming up with the most iconic locations and GMCs in your setting ("this is the diner we all hang out in, this is the chief of police we're certain is an agent of the Sleepers, this is the city park that the weird shit keeps happening in"). By the end of that session you've got a cabal and a readymade setting for the GM to start using in the campaign.