In a 5- to 6-quart pan over high heat, bring about 2 quarts water to a boil. Meanwhile, rinse green beans, trim off ends, and pull off any strings. Cut into 2- to 3-inch lengths. Add beans to boiling water and cook until barely tender to bite, 3 to 5 minutes. Drain and immerse in ice water until cold; drain well.
Fill same pan with 2 1/2 to 3 quarts water and bring to a boil over high heat. Add orzo and cook until barely tender to bite, 8 to 11 minutes. Drain and rinse with cold water; drain well.
Meanwhile, husk corn, discarding silk; rinse ears. Holding each ear upright in a large, deep bowl, cut off kernels close to the cob.
To make dressing: In a small bowl, whisk together vinegar, olive oil, shallots, mustard, tarragon, and salt and pepper to taste.
In a large, wide bowl, mix orzo with 1/2 cup of the dressing. Add more salt and pepper to taste. Spread level. Layer corn kernels, green beans, and tomatoes over pasta. Pour remaining dressing into a container. Cover and chill both salad and dressing until ready to serve.
Shortly before serving, pour three-fourths of the remaining dressing over salad and mix gently to blend. Add more dressing and salt and pepper to taste.