Heat oil in 4 or 5-quart saucepan over medium-high heat.
Add onion and saute until golden;add garlic and saute an additional 30 seconds.
Stir in wine,basil and marjoram and cook 1 minute.
Add tomatoes, increase heat and boil 5 minutes.
Reduce heat to medium,add mussels, cover and cook until shells open, about 5 minutes.
Add scallops and shrimp.
Cover and cook an additional 2 to 3 minutes, until scallops and shrimp are barely firm.
Season to taste with salt and pepper, add spaghetti and toss gently to mix.
Serve immediately, with freshly grated parmesan, if desired.
Note-to rid mussels of their sand and to plump them, scrub thoroughly and place in deep bowl.Cover with cold water and sprinkle with 2 tablespoons cornmeal. Cover and refrigerate overnight.Rinse and drain well just before cooking.