Bring 4-quart pot of water to boil; add salt. Add asparagus and cook until just tender, about 3 minutes. Transfer asparagus to bowl of ice and cold water to stop cooking. Return water in pot to boil, add peas and cook until just tender, about 1 minute. Drain in colander, then transfer to bowl of ice and cold water; let cool 1 minute. Drain vegetables and cut diagonally crosswise into 2-inch pieces. Place potatoes in pot, cover with cold water by 2 inches and bring to boil over moderately high heat; add salt. Simmer until potatoes are fork tender but not falling apart, about 15 minutes (depending on size); drain in colander. Cool under cold running water until potatoes are just warm; halve. Whisk together vinegar and mustard in large serving bowl; add oil in a slow stream, whisking until thickened and emulsified. Whisk in shallot; salt and pepper to taste. Add asparagus, peas, potatoes, and three-quarters of herbs, tossing gently to coat with dressing. Sprinkle remaining herbs over top.