Dungeon Set 3 is a type of item set. Each set consist of 5 pieces.
The pieces are not bound to specific classes, but some of the sets are considered class sets notheless. This is because they are designed for a given class, which they therefore enhance the most, which also shows in set bonuses.
The other sets seem to be more universal, meaning that they are not designed towards a specific class.
The items that form these sets are found in the lvl 68-70 Instances (Shattered halls, The Steamvault, Sethekk Halls, Shadow Labyrinth, Botanica, Mechanar, Arcatraz, and Black Morass). For details, refer to the set page.
The class sets drop from non-heroic instances, while some parts of the other sets drop from heroic instances.
This type of item set is sometimes also called "Tier 3.5".