In a small saucepan, sprinkle gelatin over 1 cup cranberry juice. Set aside to soften, 5 minutes. Place over low heat until gelatin has dissolved, 4 to 5 minutes. Add sugar; stir gently until dissolved (rub mixture between your fingers to make sure it's smooth), 2 to 3 minutes. Remove from heat.
Transfer gelatin mixture to a medium bowl; stir in orange juice and remaining 2 cups cranberry juice. Skim any foam from surface, and refrigerate until just thick enough to hold a line drawn by your finger, 30 to 40 minutes.
Arrange orange segments in the bottom of a 1 1/2- to 2-quart gelatin mold or Bundt pan; pour gelatin mixture over oranges. Refrigerate until firm, at least 2 hours.
To unmold, dip bottom of mold briefly in hot water, invert onto a plate, and shake firmly to release.