Cut a round of parchment paper to fit just inside a medium pot; set aside. Bring sugar and 3 cups water to a simmer in pot, stirring until sugar has dissolved. When liquid is clear and ubbling, reduce heat to medium-low. Add citrus slices, arranging them in a slightly overlapping layer. Cover with the parchment paper round. Place a cake pan on top of parchment to keep slices submerged. Simmer (do not boil) until rinds are almost translucent, about 40 minutes. Let cool completely in syrup. Candied citrus slices can be stored in syrup up to 1 week.
Line a baking sheet with parchment paper; set a wire rack over sheet. Transfer candied citrus to rack. Let stand until dry and slightly firm, about 8 hours.