In a large mixing bowl, combine the kidney beans, chickpeas, and green beans.

Add the cherry tomatoes, bell pepper, and red onion to the bowl with the beans.

In a separate bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and black pepper.

Pour the dressing over the bean and vegetable mixture and toss to combine.

Serve immediately or refrigerate for 30 minutes to allow flavors to meld.