This is your 3 and it gives you extra damage. I suggest activating it when you're right on top of a opponent that is low health or one that you know you can kill pretty quickly.
I advise playing Arena as everyone is together and there's not many places where enemies can go to lose you.
Use your 1 which is a jump to help chase down enemies.
You need to get all three enemies in a single match but not in a row, killing people shouldn't be that hard later in the match when you have bought some items.