The wiki claims however, that the level 3 shoulder mounts raise tower damage by 1 point. For every tower on your side.
Level 3 shoulder (really head because that's the trebuchet) increases damage of the Tower of Light on your shoulder from level 2, but not the Towers of Light that you pull from the ground with Power of the Tower.
The damage of The Rook's Tower of Light is increased.
It follows the same pattern as level 2, where adding the Tower of Light increases the damage of the Archer Tower.