In a large bowl, combine green beans, kidney beans, cannellini beans, and chopped red onion.

In a small bowl, whisk together apple cider vinegar, olive oil, sugar, Dijon mustard, salt, and black pepper to create the dressing.

Pour the dressing over the bean mixture and stir gently to combine.

Add chopped parsley and toss lightly to distribute throughout the salad.

Refrigerate for at least 2 hours (or overnight) to allow flavors to meld. Before serving, taste and adjust seasoning if needed. Serve cold or at room temperature.