Three-Headed Goblin

• WWizards of the Coast2018-01-19
  Official Ruling
  Triple strike is all the strikes (for now). A creature with triple strike deals first-strike combat damage, regular combat damage, and last-strike combat damage. Note that it has to survive each time to deal more damage later. For example, if a 3/3 creature with triple strike is blocked by a 3/3 creature with first strike, the two creatures will take each other out during first-strike combat damage. But if left unblocked, that creature will deal a total of 9 damage to the defending player or planeswalker.

Rulings via Scryfall