Cut all citrus in half. Set out either an 11x15 inch baking sheet or a 9x13 inch pan. Make sure you have an even surface in your freezer to set the tray on it.
Using a juicer ( hand or electric) squeeze the cut citrus to get the amounts of juice needed. Strain into a larger measuring cup or bowl to strain out any seeds and pulp ( you may need to strain more than once)
Whisk in the simple syrup to the juice then carefully pour into your tray or pan. You could do this with the tray or pan already placed in your freezer, so you don’t need to worry about spilling any as you walk with the tray to your freezer.
Freeze the granita, checking on it every 30 minutes until ice crystals start to form. Once its starting to freeze, scrape with a fork to fluff the ice. You will repeat this process a few times over the course of a few hours until the entire pan is scraped. Gently break up any small pieces with your fork.
Once the tray is fully scraped and fluffed, cover with plastic wrap until ready to serve.
Serve in cold glasses or bowls, garnish with a sprig of fresh mint if desired.