I hate to tell you this, but ph-3 armed planets won't work. This idea was tried long ago and the problem is that all weapons on a planet are put at it's "core", IE the center of the model. The range of the ph-3 is low enough so that the planet's radius, the surface, is out of range of the weapon. They therefore are totally ineffective. Same reason you can't put fighters or drones on a planet; they "impact" the inner surface of the planet and destroy themselves. Plasmas will fire and escape the core, however, you'll hear a nice series of collision noises as the plasma round penetrates the planet's shell as it moves on it's way to you.
I didn't design the way it works, I just play it.