In a large bowl, mix the 3 different types of beans, the celery, onion (drained of soaking water), parsley, and rosemary.
Make The Dressing:
In a separate small bowl, whisk together the vinegar, sugar, olive oil, salt, and pepper. Add the dressing to the beans. Toss to coat.
Chill & Serve:
Transfer the salad to the refrigerator for several hours, to allow the beans to soak up the flavor of the dressing. Let come to close to room temperature to serve.