Mix the ground beef, pork, pepperoni, bread crumbs, egg, and 3 tbsp of marinara in a large bowl.
Once the meat is mixed, start making your meatballs. Each one should be about the size of a golf ball. Place each of them on a foil-lined baking sheet. You should have about 14- 16 meatballs total.
After you've made all the meatballs, place the baking sheet in a preheated smoker set at a temp of 350 degrees.
After about thirty minutes (when the internal temp hits 160 degrees) remove the meatballs from the smoker, and place them in a large saucepan on the stove with 2 cups of marinara. Simmer the meatballs on medium-low heat.
While the meatballs simmer, place a store bought loaf of garlic bread in the foil package, on your smoker. Let the bread warm for about 5 minutes.
While the bread is warming, slice a half pound of mozzarella in to eight-ten slices.
Remove the bread from the smoker. Place 4-5 slices of mozzarella on the bread, followed by all of the meatballs, a few spoonfuls of marinara, and then the rest of the cheese slices.
Put the top half of the bread over all of it, place it back in the foil bag, and return the bread to the smoker for another five minutes.
Slice the sandwich into equal sized servings, and enjoy!