In a medium bowl, whisk together olive oil, lemon juice, red wine vinegar, and Dijon mustard. Add minced garlic, dried oregano, and season with sea salt and black pepper.

In a large serving bowl, combine the cannellini beans, kidney beans, and chickpeas. Add diced cucumber, halved cherry tomatoes, chopped bell pepper, sliced olives, chopped parsley, finely chopped onion, and torn basil.

Pour the prepared dressing over the mixture and gently toss to coat all ingredients without mashing the beans.

If desired, fold in crumbled feta cheese. Cover the salad or transfer to an airtight container and refrigerate for at least 2 hours, preferably overnight.

When ready to serve, gently stir the salad before plating.