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If you activate the ability of an attacking or blocking Triskelion during the declare blockers step, it will deal less combat damage as a result. If you wait until the combat damage step, but Triskelion is dealt lethal damage, it’ll be destroyed before you get a chance to activate its ability.

If Triskelion has damage marked on it, activating its ability may cause that damage to become lethal. For example, say a Triskelion with two +1/+1 counters on it has been dealt 2 damage earlier in the turn. If you activate its ability by removing a +1/+1 counter from it, it now has 2 toughness and is destroyed as a state-based action. You won’t be able to remove the other +1/+1 counter from it to activate its ability again. The first activation of the ability will still resolve.