Whisk the vinegar, oil and salt and pepper together in a bowl with a tight fitting lid, then add the green beans, garbanzo beans, red kidney beans, and onion and seal with the lid. Shake gently to coat the beans. Season, to taste, with salt and pepper, and then refrigerate overnight.
Stir in bell pepper, cucumber, mint, and parsley and gently fold in tomatoes, reserving some for the presentation. Transfer to a serving dish, arrange remaining tomatoes on top and garnish with parsley sprigs.