In a small bowl, whisk together olive oil, lemon juice, red wine vinegar, and Dijon mustard until smooth and emulsified. Add minced garlic, oregano, red pepper flakes, salt, and black pepper; mix well.

Dice cucumber, halve cherry tomatoes, dice bell pepper, slice red onion, and halve kalamata olives. Set vegetables aside in a large mixing bowl. Chop parsley and basil separately.

In a mixing bowl, gently mix together rinsed cannellini beans, kidney beans, and chickpeas.

Fold in prepared cucumber, cherry tomatoes, bell pepper, red onion, and chopped parsley.

Pour dressing over the bean and vegetable mixture, lightly toss to coat evenly.

Crumble feta over the salad mixture, gently fold in to retain its shape.

Taste and adjust seasoning as needed. Let salad sit for at least 30 minutes in the refrigerator before serving.