There is a 0 percent chance to be critted by a boss with 3 points in sanctuary. It is simply not possible to be critted, unless the boss has an extra critical strike chance (which is not the case in the current content) or the player in question sits down (which is the player's own fault). In fact a raid boss would have a theoretical -0.4% chance to critically hit you, due to them having a 5.6% chance to crit and the talent granting 6%.
I say MOSTLY uncrittable because there is still a very teeny tiny chance that you can still be critically hit by a boss. And I'm almost certain that has been documented somewhere and not just me theorizing again.