Melt 3 tablespoon butter in a medium pot over medium heat. Add the shallots, lemongrass, ginger, garlic, and Thai chile and saute until aromatic, about 2 minutes.
Add the flour and Cajun seasoning, mixing until the ingredients are coated in the flour. Add the diced potatoes and clam juice and cover the pot. Bring to a boil, reduce the heat to low, and simmer for 15 minutes or until the potatoes are just barely cooked.
Add 1 cup corn, clams, shrimp, and scallops and cover the pot. Continue to simmer until the seafood is cooked, about 7-8 minutes.
Add 1 cup coconut milk to the soup and bring it back to a simmer. Season the soup with salt. pepper, and a squeeze of lime juice. Garnish with chopped basil and cilantro. Serve immediately.