Prepare the Beans and Vegetables: In a large mixing bowl, combine the rinsed and drained black beans, kidney beans, and chickpeas. Add the corn kernels, diced red and green bell peppers, finely diced red onion, and chopped cilantro. Gently toss to distribute the ingredients evenly.
Make the Dressing: In a small bowl or jar, whisk together the olive oil, apple cider vinegar, Dijon mustard, honey or maple syrup (if using), minced garlic, salt, and pepper until well combined and emulsified. Taste and adjust seasonings as needed.
Combine and Chill: Pour the dressing over the bean and vegetable mixture in the large bowl. Toss gently until all ingredients are thoroughly coated with the dressing. For the best flavor, cover the bowl and refrigerate for at least 30 minutes (or preferably a few hours or overnight) to allow the flavors to meld.
Serve: Before serving, give the salad a good stir. If desired, stir in any optional additions like diced avocado, feta cheese, or jalapeño.