As blue on Three Way Down, you will be allied with the yellow tribe. Blue will often have to help his ally not to being doubled or even side doubled, since his ally will be a constant target, because of his smaller side hills. As such you will want to be constantly forcing red tribe back, so it may not efficiently help green achieving victory. Blue and Yellow tribe have a good front base terrain, which gives them good defence. You should avoid taking damage into your base, since terrain can get messy with it, becoming hard to recover or sucessfully defend yourself from the enemy. For that you count with good front and side defenses.
Start charging convert and cast your first on the foot of blue’s side hill and then convert the three wildmen on the front of your own base followed by converting the maximum number of wildmen on the middle passage. To get faster early build, convert on your base, but always keep an eye on your enemy to check if he does not steal you some wildmen in the middle of the map. You can always convert wildmen on the island between you and red, but that is not a very profitable use, since that island gives you almost no protection due to its size.
As blue you must not let your enemy get the erode spell kept by the stone head between yours and red tribe. This spell can be particularly harmful to your base.