Cut 2 of the lemons into quarters. Cut the remaining lemon into wedges and reserve the wedges for later use.
Fill a large pot with 12-14 cups of water. Place the two 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 10-12 minutes or until just tender.
Add the corn and sausage and cook for another 3-4 minutes.
Add the shrimp to the pot and cook for 2-3 minutes or until pink and opaque.
Drain the shrimp mixture from the pot, reserving 1 cup of the broth.
Melt the butter and whisk it into the reserved broth. Pour the broth over the shrimp mixture.
Garnish with chopped parsley and lemon wedges. Sprinkle with additional Old Bay seasoning to taste, then serve.
