Place the potatoes in a very large pot. Add 2 gallons of water and the Old Bay seasoning or Cajun spice. Bring to a boil. Add the crabs and return to boil. Boil for 6 to 7 minutes. Add the corn to the pot, return to a boil and cook 2 minutes. Drain, and then arrange the potatoes, crab, and corn on a large platter. Garnish with lemon wedges. Serve with small pots of melted butter for dipping the crab, if desired.
Note: Old Bay seasoning is available in the seafood department or the bottled spice aisle. Cajun spice is available in the bottled spice aisle.