Mythic Path ability Fickle Attack should allow stacking to a limi of 3 - it doesn;t, though the Description text is correct.
Fickle Attack Type: MythicPathAbility Requirements: all of ( at least 1 Mythic Path ~ Trickster Special Ability ) and var: MythicLevel at least 3 Description: Whenever you roll damage for a melee or ranged attack with a weapon or alchemical item, you can treat any natural 1s on the damage dice as if they were the highest possible number on those dice. You can select this ability up to three times. The second time you select it, treat 1s and 2s as the highest value. The third time, treat 1s, 2s, and 3s as the highest value.
Pathfinder core rulebooks (set)