The first of five Assassin's Creed III DLC should be up today (European PSN owners you will have to wait till tomorrow and the DLC for Wii U will also be available at a later date). It will be automatically available if you bought the season pass (Doesn't appear to say how much it is on it's own). This DLC will grant you basically all the stuff that was pre-order bonuses, and it would appear that the next DLC titled "Battle Hardened" will exclusively focus on multiplayer. As for specifically what is in "Hidden Secrets":

• Lost Mayan Ruins Mission
• A Dangerous Secret Mission
• Ghost of War Mission
• Colonial Assassin Outfit for Single Player
• Captain of the Aquila Outfit for Single Player
• Redcoat Multiplayer Character
• Sharpshooter Multiplayer Character

Update: The DLC is available right now for season pass owners only, will be out for everybody else on Dec. 11th.

The only thing I care about is getting that mad king George Washington DLC.

Is 3 MB too large to suspect on-disc DLC?

This is the pre-order/store exclusive content mostly. I'd feel kinda bummed if I paid for a season pass and then see that the 'content' in 1 of 5 promised 'packs' is 'filler' from pre-released bonuses that I may have already got if I bought the game from X store.