1. Rinse and drain the chickpeas, cannellini beans, and black beans well, then let them sit for a few minutes so excess water drains away.

2. Add the beans to a large mixing bowl with the diced cucumber, halved cherry tomatoes, chopped red bell pepper, chopped red onion, chopped celery, chopped parsley, and crumbled feta.

3. In a small bowl or jar, whisk together the olive oil, red wine vinegar, lemon juice, Dijon mustard, minced garlic, dried oregano, salt, black pepper, and crushed red pepper flakes until smooth.

4. Pour the dressing over the bean mixture and toss gently until everything is evenly coated.

5. Cover and refrigerate the salad for at least 30 minutes so the flavors can blend and deepen.

6. Toss once more before serving and adjust seasoning if needed with extra salt, lemon juice, or vinegar.