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 the frozen baby peas and cook for 20 seconds longer, until the sugar snaps and snow peas are crisp-tender and the baby peas are heated through. Drain and immediately transfer the colander to the ice water to stop the cooking. Drain again and pat the peas dry. Add the peas to the dressing, season with salt and pepper and toss to coat. Serve.