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.

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.