The third test is to catch a Rock Turtle, which come up to the West Gate of Horizon quite often. Aha, I know these things. Usually there very docile but if you touch them, they get angry and start biting people. You better not underestimate them.
After accepting the quest, take the road west out of Horizon. Follow the road until it forks. The Rock Turtles can be found in the field west of where the road forks. Kill the required number of monsters and then return to Guild Official Oveli.
You've passed the third test! Let's hurry up and finish the other two as well!