Cook string beans in boiling salted water until fork tender (then put in ice bath), drain until dry.
In a large pasta bowl add string beans, chickpeas, kidney beans, garlic, oregano, onion, olives, celery, basil, parsley. Add salt, red and black pepper to taste. Mix well.
Whisk together olive oil, lemon juice, vinegar, toss with salad until combined. Let sit for 1/2 an hour, mixing every 10 minutes to get all ingredients coated with mixture.
Chill for at least 1 hour and serve, salad can be made 1 day ahead of time.