To begin your search for the third trail treasure, you will need to find the trail that circumnavigates a manmade body of water. When you reach the river delta walk the plank over untroubled waters. You are standing on the treasure. Here’s a map to help you:
Also, if you haven’t completed Treasure Hunts #1 and/or #2, you have up until the entire Trail Challenge is over to find and turn in those treasures.