Start by draining and rinsing all the beans thoroughly under cold water. Place them in a large bowl — the kidney beans, chickpeas, and black beans
Dice the bell pepper, chop the onion finely, and halve the cherry tomatoes. Add them to the bowl along with the corn and cucumber.
In a small jar or bowl, whisk together the olive oil, vinegar, lemon juice, honey, mustard, garlic, salt, pepper, and chili flakes (if using). Shake or whisk until the dressing is smooth and slightly thickened.
Pour the dressing over the beans and vegetables, and toss everything together until well coated. Let it sit for 10–15 minutes before serving so the beans can soak up the dressing — this step makes a big difference in flavor.
Right before serving, stir in the chopped parsley (or cilantro) and sprinkle feta cheese on top if you’re using it.