"Why do bad things always come in threes?"
—Gisulf, expedition survivor

Triskelion

Triskelion enters with three +1/+1 counters on it.
Remove a +1/+1 counter from Triskelion: It deals 1 damage to any target.

March of the Machine Commander

Triskelion enters the battlefield with three +1/+1 counters on it.
Remove a +1/+1 counter from Triskelion: It deals 1 damage to any target.

Optimally designed for agility, mobility, and lethality.

Dominaria Remastered

"What's got three legs and three heads and wants to kill us? That's not a riddle!"
—Jayeen, Fallaji scout

The List

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.