1. Mix mushrooms, dressing, onion flakes, Adobo seasoning and pepper together in a bowl. Divide into thirds. Place different ground meats in 3 different bowls. Add 1/3 of the mushroom mixture to each. Shape into 6 patties (2 per flavor, about 3/4 inch thick) and refrigerate until grilling.
2. Heat grill to medium-high heat for 5 to 10 minutes. Add patties and cook 5 minutes. Flip over, and continue to cook for an additional 5 minutes, or until instant-read thermometer registers 150 degrees F for beef, 160 degrees F for turkey and 145 degrees F for pork. Add cheese, if using, during last minute of cooking.
3. Place burgers on buns or bread; top with lettuce and tomato.