The 3rd Gate is a location in Xenoblade Chronicles. It is the final gate that can be opened at Sword Valley, and is located north of the Ether Storage Area. Beyond it is an area which ends in another gate, presumably the 2nd Gate; however, it cannot be opened. The switch for the 3rd Gate can be found in Enalda Control Base.
A small army of Mechon is located here, and a surprise quest obtained just as the 3rd Gate is passed, requesting that the largest Mechon, the Reinforcement M104, is defeated. The best way to accomplish this is to lure away the smaller groups of Mechon and defeat them one by one, as the party will have no chance of beating them all at once. After completing this quest and defeating the Reinforcement M104, the Unique Monster Mischievous Naberius will spawn.