The areas in Act 3 are:

Main Town

Spider Forest - you see the Dark Wanderer briefly before he vanishes. You have jungle hunters and cloud stalkers (flying things) harassing you here. Mostly it's really annoying because you have to walk along rivers and forests, it's all very wiggly.
--> Arachnid Lair is within the forest, with flame and poison spiders. 1 level of annoying little passages.
--> Spider Cavern is also within the forest, with more spiders, giant lampreys and fiends.

Great Marsh is next, with bramble hulks, drowned carcasses and swamp dwellers.

Flayer Jungle follows.

Diablo 2 Walkthrough
Walkthrough Master Index