Our version of three-bean salad, a perennial picnic favorite, is made by tossing haricots verts, kidney beans, and cannellini with a tangy honey-mustard vinaigrette.
Make the vinaigrette: In a small bowl, whisk together lime juice, tarragon, chives, honey, mustard, and salt until combined. Set aside.
Prepare an ice bath; set aside. Bring a medium saucepan of water to a boil. Add haricots verts or green beans; simmer until bright green but still crisp, about 2 minutes. Using a slotted spoon, transfer haricots verts to ice bath, and let cool completely. Drain in a colander, and pat dry with paper towels. Place in a large serving bowl. Add kidney beans and cannellini, and drizzle with vinaigrette. Toss to coat. Gently toss in the mache or watercress; season with pepper. Serve.