Dissolve lime gelatin in 1 c. hot water; add grapefruit juice. Pour into a shallow pan; chill. Dissolve cherry gelatin in remaining 1 c. hot water; add orange juice. Pour into a shallow pan; chill.
Soften unflavored gelatin in ¼ c. cold water. Heat pineapple juice to boiling; add softened gelatin and stir until it is dissolved. Chill until mixture thickens (a little thicker than unbeaten egg white). Add sugar and vanilla to whipped cream and fold into pineapple-gelatin mixture.
Cut lime and cherry gelatins in small cubes with a knife. Fold into pineapple and whipped-cream mixture, saving out a few cubes of both colors.
Spoon into pie shells. Chill. To serve, garnish with reserved gelatin cubes and, if desired, with whipped cream.