Cook shrimp in a 4-quart saucepan of boiling salted water until just cooked through, 1 to 2 minutes, then drain in a colander and cool.
Cut peel and white pith from limes with a sharp knife and discard. Cut lime segments free from membranes, then finely chop enough segments to measure 1/4 cup. Mince enough lemongrass from bottom 6 inches of stalks to measure 2 tablespoons total.
Stir together chopped lime, minced lemongrass, lime juice, bell pepper, scallions, chiles, fish sauce (to taste), sugar, and salt until sugar is dissolved. Toss shrimp, tomatoes, and cucumbers with dressing and top with herbs.