Prepare the Beans: Open the cans of chickpeas, cannellini beans, and kidney beans. Empty them into a large mesh colander in the sink. Rinse thoroughly with cold water until clean and no foam remains. Let drain completely for 5 minutes.
Chop the Vegetables: Wash and finely dice your cucumber, bell pepper, and red onion into uniform pieces roughly the same size as your chickpeas. Cut the cherry tomatoes into halves or quarters.
Mix the Dressing: In a small bowl or jar, whisk together the extra virgin olive oil, fresh lemon juice, grated garlic, dried oregano, sea salt, and black pepper until completely emulsified.
Toss the Salad: In a large serving bowl, combine the thoroughly drained beans, diced cucumbers, tomatoes, bell peppers, onions, sliced Kalamata olives, and chopped fresh parsley. Pour the emulsified dressing over top and toss thoroughly until every ingredient is glossed.
Add Cheese: Gently add the crumbled feta cheese and fold it in lightly with a spoon to prevent it from breaking down too much or turning muddy.
Chill and Serve: Cover the bowl securely and chill in the refrigerator for at least 30 minutes before serving to let the rich Mediterranean flavors fully mature.