Preheat your oven to 375°F (190°C). Cook pasta shells according to package directions until al dente; drain and toss with olive oil.
Sauté diced onions in olive oil and butter until translucent, then add garlic, shrimp, and lobster until cooked through. Mix in flaked crab meat off heat.
In a bowl, combine cream cheese, ricotta, half of the Parmesan and mozzarella with the seafood mixture. Chill for at least 30 minutes.
For the sauce: Melt butter in a saucepan, whisk in flour, then gradually add warmed milk while stirring to create a smooth sauce. Stir in seasonings and remaining cheeses.
Spread some sauce in a baking dish, fill each shell with the seafood mixture, place them in the dish, cover with remaining sauce and cheese.
Bake covered with foil for about 25 minutes; uncover for an additional 15-20 minutes until bubbly and golden. Let rest before serving.