Steam the green, wax, and lima beans 10 minutes, or just until crisp-tender. Plunge into cold water and drain. Place in a bowl with chickpeas and onions.
Whisk together the oil, vinegar, and mustard, and add salt and freshly ground pepper to taste.
Drizzle over the beans and onions and toss to coat. Salad can be served at room temperature or chilled.