In a large bowl, combine wax beans or black beans, green beans or lima beans, red kidney beans, sweet pepper, and red onion.
For dressing, in a screw-top jar, combine vinegar, sugar, oil, celery seeds, dry mustard, and garlic. Cover and shake well. Pour over vegetables; gently stir. Cover and chill for at least 4 hours or up to 24 hours, stirring often.