Put one-third of seasonings in an 8-qt. pot; fill with water and bring to a boil. Set a big bowl of ice and water near stove.
Add 3 lbs. shrimp and cook just until pink and curled, 1 to 2 minutes. Lift to ice water to cool. Drain and chill in paper towel–lined sealable bags. Repeat twice, using fresh seasonings each time.