Bring 3 cups water and sugar to a boil in a large saucepan, stirring until sugar dissolves. Remove pan from heat; add tea bags, and steep 5 minutes. Remove tea bags with a slotted spoon (do not squeeze).
Cool tea to room temperature; stir in orange juice, lemon juice, and remaining 3 cups water. Cover and chill at least 1 hour. Serve over ice; garnish with orange slices, if desired.