Add the butter, onions, leeks and celery to the pot and cook for 3 to 4 minutes, stirring often, until the vegetables are soft and translucent. Add the garlic and cook for another minute. Sprinkle in the flour and cook for 2 minutes, stirring constantly. Stir in the fish stock, milk, cream and reserved clam juice, then increase the heat to high and bring to a boil, stirring often. Once boiling, reduce the heat to low, toss in the potatoes and simmer, uncovered, for 15 minutes. Add the clams, reserved bacon, haddock, crab meat, salt and pepper and simmer for another 5 minutes. Taste and adjust the seasoning.