1In medium bowl, pour 1 cup of the boiling water on lemon gelatin; stir until gelatin is dissolved. Slowly stir lemon yogurt into lemon gelatin until blended. Pour yogurt mixture into 12-cup mold. Refrigerate 1 hour or until set.
2In large bowl, pour remaining 2 cups boiling water on orange gelatin; stir until gelatin is dissolved. Stir in 1 1/2 cups of the cold water and the mandarin oranges. When lemon gelatin is set, carefully pour orange gelatin mixture over lemon gelatin. Refrigerate 1 hour or until set.
3Drain pineapple, reserving juice in 2-cup microwavable measuring cup; set pineapple aside. Add enough water to pineapple juice to measure 1 cup; microwave on High 2 minutes or until boiling. In large bowl, pour boiling liquid on pineapple gelatin; stir until gelatin is dissolved. Stir in remaining 1 cup cold water and reserved pineapple tidbits. When orange gelatin layer is set, carefully pour pineapple gelatin over orange gelatin layer. Refrigerate at least 2 hours or until set. Unmold on serving plate. Cover and refrigerate any remaining gelatin.