This third Message in a Bottle can be found in Hayter's Folly after killing Sandman. You'll walk past Sandman's house and off to the right will be a waterfall and an elevator which previously didn't work. If you drop off of the waterfall and onto the next ledge just below you, you should be able to find the bottle next to a banjo.
|Normal||True Vault Hunter|
Gear: Assault Rifle
After getting this message, follow the waypoint to another part of the cave. It will take you to Sandman's Overlook which is the area with a camp, a few waterfalls and a bunch of small pools of standing green water. In this camp, you need to climb up some stairs to the outer rim of the camp. You'll follow this around to a set of two waterfalls. The first falls into a stream that very quickly leads to a second waterfall. Just past this is a small ledge below with a bunch of mushrooms that will take you behind the next waterfall.
Behind one of the waterfalls, tucked into the wall will be a small green glass bottle similar to the one you found the message in. This one will be turned sideways and wedged between the rocks as if propping them up. If you investigate the bottle, the wall will fall away and reveal a hidden room. Inside this room, you'll be attacked by a Cave Crystalisk which can be tough to deal with in this small room so back up into the previous room if you need to. In this room, look for the giant black X at the back. Dig that up and open the chest to finish the mission.