I thought AP had increased critical chance, not critical severity. Guess I need to look at the description again . Hmm, I have yet to see a purple mark XII, but I've seen [CrtH]x3 Mk XI tetryon DHCs before, my old fleet leader had one. He said he bought it for about 40 million EC. That's a little rich for me...most I've ever had on a character was 3 million.
I saw a Phaser Dual Cannon (non Heavy) CritHx3... once. I debated it for too long someone got it for around 40 mil too. lol
Honestly a good way to go if you can find a set is MK XII CritHx2 Blue... they would be the same dmg as the MK Xi Emblem Polarons. If you can find 2-3 of those your set.