How do you unlock the artifact in thing thing arena 3

To unlock Artifact 19, you must earn all of the weapon achievements. You are not required to earn them all in a single round, though. A complete list of ALL achievements can be found here: http://www.crazymonkeygames.com/guide/Thing-Thing-Arena-3//

The only achievements you need to earn are the "weapon bonuses," with the obvious exception of the Artifact 19 Bonuses. During the round in which you earn the final achievement, you MUST die, as going to the main menu from the pause menu will not save your progress. Once you do this, Artifact 19 will be equipped as the third weapon during any subsequent rounds. It does not have ammo, instead it will "recharge" and you also cannot dispose of it with the space button. Good luck!