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If I have a Laser Cutlass and a Crushing Treads on say, Slammer, and I attack an opponent, does Slammer get the pierce 3 from the Laser Cutlass plus the pierce 3 from the Crushing Treads since pierce adds together and both pierce abilities are triggered differently? So, attacking for 4 with Pierce 6.

Thats correct. In this scenario you would be attacking for 4 with Pierce 6. Pierce stacks on Pierce, but not on regular attack damage.
Source: Wizards of the Coast (2019-08-23)    【Link to Ruling】
Tags: Pierce