Finely grate the peel from 2 oranges; set aside. Cut thin strips of orange peel from the remaining orange for garnish; set aside. Squeeze juice from all oranges. Cut onions to separate the white and green parts. Thinly slice the white parts; cut green parts into 1 inch lengths for garnish.
For sauce, in a small bowl, combine sugar and cornstarch; stir in orange juice until smooth. Stir in the soy sauce, garlic, ginger, grated orange peel and white parts of onions; set aside.
In a large nonstick skillet or wok coated with cooking spray, stir fry beef in oil until no longer pink. Stir the sauce and add to the pan. Bring to a boil; cook and stir for 2 minutes or until thickened.