In a large mixing bowl, combine garbanzo beans, kidney beans, pinto beans, and onion. Toss gently to mix everything evenly.
In a small bowl, whisk together apple cider vinegar, sugar, oil, salt, and pepper until the sugar is mostly dissolved.
Pour the dressing over the bean mixture and toss gently until everything is well coated.
Cover the bowl with plastic wrap or transfer to an airtight container. Refrigerate for at least 4 hours, or overnight, to allow the flavors to meld.
Before serving, give the salad a final toss and sprinkle with parsley for garnish.
Serve chilled or at room temperature.