Bring seafood stock to a boil in a medium saucepan. Add grits, reduce heat, and cook until grits are soft, about 20 minutes. Remove from heat and stir in butter, scallions, tomatoes, parsley, and the 3 cheeses. Use milk to adjust the texture of the grits mixture if it is too thick. Gently fold in crabmeat and transfer to serving dishes. Garnish with parsley sprigs.