I'm not sure if this is a bug or if it is just a flaw in the design of the Gun Tower. When there is only a single unit within the range of the Gun Tower only one of it's defensive turrets target the unit and fire while the other two turrets remain inactive. It just seems like it would make sense if all three turrets could target a single unit if no other enemy troops were nearby.
It could just be a glitch as there is supposed to be a single unit sized gap between the tower and its attack zone.
Drop a jugg at a neighboring building to the gun tower and watch as only one turret will target him. Like I said, I'm not sure if this is a bug or if the gun tower was designed so that it wouldn't focus all 3 turrets on a single target.
For that aspect I've never seen it fire all 3 at one unit, even when there was only one unit left. So I'm voting design.
I've known about this, I thought it was by design. I thought the ship was the same way before the patch, but I think (haven't very actively tested..) but it seems to me that now the ship can fire all cannons at the same target.