This is definitely a case of 'that's not what it says, but...' for weapons without an inherent AP, it would make more sense as AP 3X, meaning where an Armour Piercing round does AP X - so a weapon that did 4D would have an AP 4, a directed plasma would instead be AP 3 times 4, or 12. Same would go for Plasma (2X, not x2). Not an official answer - I just copied forward what was in the first edition into the update, and obviously didn't think deeply about it until... now.
So what to do with a gauss rifle (4D damage) that is already AP5? Well it would either be:
AP 5x3 = 15
AP 4x3 = 12
AP 5 + 4x3 = 17
I could make a logical case for any of those, but for simplicity and following the RAW it would be 15. But that's just my opinion. For a directed plasma shotgun slug, by RAW it would be AP 0, but that makes little sense, so...
Referee: The guy behind the counter at Eneri's Weapons Emporium says: "Yeah, sorry, we don't stock none of them plasma slugs. Maybe try Sharik's House of Mass Destruction down on Level C." But your Starport Guide shows Sharik's wrapped in police tape with a lot of blast marks around what's left of the storefront.