If I remember correctly 1 - deck gun, 2 - forward gun and 3 - aft gun. I’m not sure about the 3 as it is a long time since I bothered getting an aft gun as the avid suppressor is too useful for me.
As said, the three gun slots are mapped to 1, 2, and 3. The other number keys can be used for the combat items like Blue Scintillack or Flares - they can be rearranged on the Hold tab of the Gazetteer, below the ship diagram. edited by Optimatum on 1/10/2017