In a baking dish, combine the teriyaki sauce, soy sauce and garlic. Make 1/4-inch-deep cuts about 1/2-inch apart on both sides of the steak, cutting diagonally across the grain. Rub both sides with the mustard and place the steak in the marinade. Refrigerate for up to 3 hours, turning occasionally. Preheat the broiler and line a shallow pan with foil. Broil for 6 minutes on each side for medium-rare, basting once on each side with remaining marinade. Let rest before slicing across the grain.