1. In a small saucepan, bring 1 cup of water to a boil, add the sugar and let it simmer until it dissolves. Let it cool, then add the Campari and lemon juice. Refrigerate until serving.
2. Cut the peel of the grapefruits and oranges with a serrated knife. Remove the tops of the fruits in a bowl by sliding a small peeling knife between the flesh and the membranes. Mix the fruits with the Campari and refrigerate.
3. Divide the salad into four plates and garnish each plate with chopped mint.