In a small bowl, mix the lime juice, oil, fish sauce, chiles, shallot, and sugar. Season the dressing to taste with salt and black pepper.
In a large bowl, combine the shrimp, cucumbers, and tomatoes. Add the dressing, mint, and basil and toss to combine. Divide the salad among 4 plates. Sprinkle with peanuts. Serve right away with lime wedges to squeeze over the salad.