Vilgax had the three bounty hunters Kraab, Sixsix, and Tetrax audition to see which one should pursue and retrieve the Omnitrix. They had to fight many of the basic drones and a large "mantis" drone to pass. If they survive the encounters with the robots then they are hired, if they do not then they are not hired. All three of the bounty hunters survived so all three of them were hired.