Fill a large skillet with deep sides with water about halfway and bring it to a boil on the stove. Salt it generously. Boil the Lobster Tail (4) in it for 8-9 minutes, until the shells turn a vivid red. Remove them and let them cool enough to handle.
When you can handle the lobster tails, crack the meat out of the shells and dice it into bite-sized chunks. Drain the water from the skillet you boiled the tails in and put the skillet back on the stove.
Heat Butter (4 tablespoon) in that skillet and add the Garlic (3 clove). Let it get fragrant for about a minute.
Add the lobster meat in next and let it cook for a minute or two in the butter and garlic. Remove the lobster to a plate and set it aside.
Add the remaining Butter (6 tablespoon) and the All-Purpose Flour (6 tablespoon) to the pan. Whisk them together until they become a thick, golden paste for about a minute.
Slowly pour in the Milk (3 cup) while you keep whisking to bring the béchamel sauce base together. Season it with a Salt (to taste) and Ground Nutmeg (1/4 teaspoon). Reduce the heat to low and let it bubble and thicken for 10 minutes, whisking it often.
While the sauce cooks, boil the Medium Pasta Shells (1 pound) in another pot of boiling water. Salt the water generously before you do. Once it is tender after 8-10 minutes, drain it.
Whisk the Gruyère Cheese (4 ounce), Fontina Cheese (4 ounce), and Extra Sharp Cheddar Cheese (4 ounce) into the sauce until they melt right in, then add the lobster meat back in. Finally, add the cooked pasta in and stir everything together thoroughly.
Take the pan off of the heat and serve the mac and cheese immediately!