Zest all 3 oranges directly into a medium bowl, taking care to remove only the bright orange layer and none of the white pith beneath, which turns the syrup bitter.
Add 2 cups of sugar to the bowl and use your fingers to press and rub the zest into the sugar for about 1-2 minutes, until the mixture looks damp and fragrant.
Pour in 1 cup of boiling water and stir until all the sugar has fully dissolved, about 1-2 minutes.
Juice the 3 zested oranges and stir the juice into the syrup. Let the mixture sit uncovered for 15-30 minutes to infuse.
Pour the syrup through a fine mesh strainer into a clean jar or bottle, pressing on the solids to extract all the liquid. Discard the solids.
To serve, add 1 tablespoon of syrup to a glass filled with ice and top with sparkling water. Taste and adjust the amount of syrup to your liking.
