Watch Llama's video on act 3 maps.Spider Forest, Great Marsh, and Flayer Jungle are all 2x6 tiles in size with the river running the whole length (maybe turning a few times) and 3 side areas each. They connect to each other at the 3rd, 5th, or 6th tile. Flayer Jungle can be off the Spider Forest (good) or the Great Marsh (bad).
And as far as side areas, if you're using the map reveal you can tell them apart like so:
* Spider Forest has Arachnid Lair with waypoint, Spider Cavern by itself, and one empty side area.
* Great Marsh has a waypoint and 2 empty side areas.
* Flayer Jungle has Swampy Pit with waypoint, Gidbinn with Flayer Dungeon, and one empty side area. It also *always* turns at the end just before the entrance to LK.