i d keep one at the top so that the water is aerated, its good to have a brisk flow at the top, but not so that water is spraying out of the tank.
the second i would have behind your LR so that debris doesnt settle back there, and the third just a current in the middle of the tank. id put one on the left side and two on the right side, or vice versa just so the water isnt moving in one direction.