Preheat your waffle iron to medium heat (or to the setting recommended by the manufacturer).
In a large bowl, whisk the 3 large eggs until combined. Add 1 cup water and 1/3 cup vegetable oil and whisk until smooth.
Pour the entire 1 (16.5 oz) package of Devil's Food Chocolate Cake Mix into the bowl and stir until just combined and no large dry streaks remain. Do not overmix.
Lightly spray the preheated waffle iron with non-stick cooking spray.
Pour the amount of batter recommended by your waffle iron’s manufacturer onto the iron, spread slightly if needed, and close the lid.
Cook according to your waffle iron’s instructions (or until the waffle is set and the steam has mostly stopped), then carefully remove the waffle.
Repeat with remaining batter. Let waffles rest 1–2 minutes before serving so they finish setting.