Bring 6 quarts water and 1 tablespoon salt to a boil in a large stockpot. Plunge 3 lobsters, head first, into boiling water; return to a boil. Cover, reduce heat, and simmer 10 minutes for the first pound and 3 minutes for each additional pound. Remove lobsters with tongs; set aside. Return water to a boil; repeat procedure with remaining lobsters.
Cook potatoes with remaining 1 teaspoon salt in boiling water to cover in a large Dutch oven 5 minutes. Add corn; cover and cook 5 more minutes or until potatoes are tender.
Arrange lobsters, potatoes, and corn on a serving platter. Serve with melted butter, lemon halves, and baguette slices. Add parsley to serving plates, if desired.