1. Bring a large pot of salted water to a boil. Add the green beans and cook until they are crisp-tender, about 5 minutes. Drain and rinse the green beans under cold water until they are chilled; pat the green beans dry.
2. In a large bowl, whisk the mustard with the vinegar. Gradually whisk in the olive oil and season with salt and pepper. Add the green beans, tarragon, chives and thyme and toss to coat. Add the tomatoes, toss gently and serve.