Drain and gently rinse all three canned beans until packing liquid is gone.
Drain 2 minutes, spread over clean towels, blot and stand 5 minutes.
Thinly slice red onion, finely dice celery and chop parsley; blot soaked onion if used.
Whisk vinegar, sugar, Dijon, celery seed, salt and pepper for 30 to 45 seconds.
Stream in oil while whisking until droplets are small and evenly dispersed.
Fold onion, celery and kidney beans into the dressing.
Add green beans, wax beans and parsley; fold gently with a broad spatula.
Cover and refrigerate 4 to 24 hours, folding once after the first hour.
Taste chilled salad and adjust vinegar by the teaspoon, sugar by one-half teaspoon or salt by a pinch.
