In a large pot, bring water to a boil and cook the potatoes until tender, about 10 minutes. Add corn and cook for another 5 minutes. Drain and set aside.
In a large bowl, combine shrimp, crab legs, clams, cooked potatoes, and corn. Add melted butter and Cajun seasoning. Toss to coat evenly.
Place the seafood and vegetables on the grill. Cook for about 10-15 minutes, turning occasionally, until the seafood is cooked through and has a nice char.