By Rules As Written (RAW):
From a single shot, damage is applied together.
In your example above, that would be 3d10+9.
If you're firing more than one weapon (ie. a macro battery and a macro broadside), you can choose (before you roll to hit) if you want them to fire a single volley or as seperate volleys.
Either way, damage is totaled for the entire volley, and each volley can cause a maximum of 1 critical hit.
This is in the core book. It's also surprisingly bad design, because it largely invalidates every weapon besides macrocannons.
Several fixes exist. My local houserule is that multiple weapons cannot be grouped into volleys. This is a simple fix and solves the problems most of the time in my experience.
Someone on these boards have proposed a different system, which make ship-weapons work more or less like personal scale weaponry. I'm sure someone else will be along shortly and provide you with likes to this.