In a large mixing bowl, combine the green beans, wax beans, kidney beans, red onion, bell pepper, and celery.
In a separate medium bowl, whisk together the granulated sugar, apple cider vinegar, vegetable oil, salt, and black pepper until the sugar is fully dissolved.
Pour the dressing over the bean mixture and toss thoroughly to coat all ingredients.
Fold in the minced parsley.
Cover the bowl with plastic wrap and refrigerate for at least 4 to 12 hours to allow the flavors to marinate.
Toss once more before serving cold.