Cook frozen beans according to package directions; drain. Meanwhile, in a large bowl combine wax beans, red kidney beans, sweet pepper, and onion. Add green beans.
For dressing, in a screw-top jar combine the vinegar, sugar, salad oil, celery seeds, dry mustard, and garlic. Cover and shake well. Pour over vegetables; stir lightly. Cover and chill for at least 4 hours or up to 24 hours, stirring occasionally. Serve with a slotted spoon. Makes 6 side-dish servings.