In a large bowl, combine the cantaloupe, honeydew and strawberries. Drain pineapple, reserving 1/3 cup juice; set juice aside. Add pineapple to fruit mixture; cover and refrigerate until chilled.
For dressing, in a blender, combine the sugar, onion, salt, mustard and reserved pineapple juice; cover and process until blended. While processing, gradually add oil in a steady stream. Stir in poppy seeds. Cover and refrigerate until chilled. Serve with fruit salad.