In a large bowl, combine all three types of beans, red onion, celery, and parsley. Toss to combine.
1 (15 ounce) can chickpeas, 1 (15 ounce) can red kidney beans, 1 (15 ounce) can Cannellini beans, 1/4 medium red onion, 4 stalks celery, 1 cup fresh parsley
In a large liquid measuring cup, whisk together the vinegar, olive oil, sugar, rosemary, Dijon, salt, and pepper. Pour this over the bean mixture. (I do recommend using it all.) Toss well to combine. Sample and season with more salt and pepper to taste.
1/3 cup apple cider vinegar, 3 Tablespoons extra-virgin olive oil, 1-2 Tablespoons white granulated sugar, 1 teaspoon dried rosemary leaves, 1 teaspoon Dijon mustard, 1 teaspoon kosher salt, ¼ teaspoon black pepper