Take a lime wedge and run it around the rim of a rock glass or martini glass and gently dip it in salt.
In a cocktail shaker, add the lime juice, orange juice, lemon juice, syrup, and a few ice cubes. Stir and close the lid. Shake vigorously for a few seconds.
Take the cap off the lid and pour the drink into the glass. Add a little tonic water, stir, and garnish with a slice of lime.
Serve immediately.
Notes
Making a big batch? Store in a pitcher and refrigerate for up to a week.