Stir sugar into boiling water and whisk until dissolved. Remove and chill until cold.
Add watermelon, lime juice and 1-2 tablespoons simple syrup to the blender and blend until smooth. Fill 6 popsicle molds ⅓ of the way full. Pop into the freezer for 40 minutes.
Rinse out blender and add honeydew, 1-2 tablespoons simple syrup, and lemon juice. Blend until smooth. Add to a container to chill.
Rinse out blender once more and blend cantaloupe, grapefruit juice, and 1-2 tablespoons simple syrup. Remove to a container to chill.
After the watermelon is semi-firm, add honeydew to the mold. Place in freezer for 40 more minutes, then top with the cantaloupe. Place in freezer until completely firm, another 4 hours.
Recipe by 5* trending recipes with videos at https://food.theffeed.com/2017/05/10/melon-rainbow-popsicles/