Changing Crit Multipliers

[Gunner-Recall]

Hello all, I may have missed this, but I am working on a character, and I wish to use a rapier. Is there a way to change the crit multiplier from X2 to X3? This could be simple and I just missed it, however, I have been looking and have not seen anything.

Thanks!

[Artanthos]

Kensai using perfect strike

Fighter or kensai using weapon mastery

Kensai using weapon mastery and perfect strike can change the multiplier to x4.