Thomas is right - 3rd graders are old enough to set their own goals.
You can guide them by giving them examples or choices (at this age they may try to pick what they think you want them to pick). Or you can set an example by making a goal for yourself.
My suggestion is that the goals focus on improving or increasing skills rather than win\lose situations. For instance, test them on a physical skill like sit ups in one minute and then challenge them to increase that by 35% (I never went to camp so I hope this is not a stupid example). If they need to do community service, they can choose the project and goal. Use your influence to help them make the goal realistic.
