Cut 2 of the lemons into quarters. Slice the remaining lemon into wedges and reserve the wedges for garnish.
Fill a large pot with 14-16 cups of water. Place the 2 quartered lemons in the water along with the Old Bay seasoning, garlic and onion. Bring to a boil.
Add the potatoes to the pot and cook for 8 minutes.
Add the lobster tails and cook for 5 minutes.
Add the clams, crab, and corn, and cook for 5 minutes.
Add the shrimp and sausage and cook for 2-3 minutes.
Drain the seafood mixture from the pot, reserving 1 cup of the broth.
Melt the butter in a small bowl and whisk it into the reserved broth. Pour the broth over the seafood mixture.
Garnish with chopped parsley and lemon wedges. Sprinkle with additional Old Bay seasoning to taste, then serve immediately.