Bring a large saucepan of salted water to a boil. Fill a large bowl with ice water. In another large bowl, whisk the shallot, vinegar, and sour cream together. Whisk in the olive oil until emulsified. Season with salt and pepper.
Add the sugar snap peas to the boiling water and blanch for 20 seconds. Add the snow peas and cook for 20 seconds. Add frozen peas and cook for 20 seconds longer, until sugar snap and snow peas are crisp-tender, and frozen peas are heated through. Drain and immediately transfer the colander to ice water to stop the cooking. Drain again and pat the peas dry. Add peas to dressing and season with salt and pepper. Toss to coat.