Cut the watermelon off the rind and then cut into chunks.
Place in a food processor, chopper, blender. I used the Ninja and it really makes a smooth job of this process. Here is a link to the exact one that I have - Ninja Master Blender.
After you have the watermelon all blended - pour it through a sieve. The amount will equal about 4 cups. Do not worry about the exact measurement. If you get 6 cups it will be a pretty sweet popsicle but why not.
Squeeze all the juice you can out of the grapefruit. I use one of those little wooden-handled reamers as shown in the citrus photo above.
Pour this liquid through a sieve also. You want about 2 cups of grapefruit juice. Again, don't stress about exact measurements and if it's less you'll just have sweeter popsicles. Set aside.
Next get all the juice from the oranges. I used the reamer again.
Put through a sieve and this should measure about 1 cup of orange juice.
Mix all three of the juices together and stir. Taste the juice. If it isn't as sweet as you think your family will like then add a tablespoon of maple syrup. It's the grapefruit flavor that you're working with. Add maple syrup one tablespoon at a time and taste each time. Don't make it too sweet.