Kebobs: Put the salmon, halibut and tuna in a resealable plastic bag. In a medium bowl whisk together the olive oil, lemon zest, lemon juice, garlic, parsley, thyme, salt, and pepper. Pour the oil mixture over the fish and seal the bag. Refrigerate for 30 minutes. Thread each skewer with 2 pieces of salmon, 2 pieces of halibut and 2 pieces of tuna, alternating the fish on the skewers.
Butter: In a small bowl, mix together the butter, garlic and parsley. Season with salt and pepper, to taste. Transfer the butter to the center of a 6-inch-square piece of plastic wrap. Fold the plastic over the butter and form it into a log, about 4-inches long and 1 1/2 inches in diameter. Refrigerate until firm, about for 1 hour. Slice the butter into 1/4 to 1/2-inch thick slices and transfer them to a plate.
Put a grill pan over medium-high heat or preheat a gas or charcoal grill. Season the fish with salt and pepper, to taste, and grill the skewers until opaque, about 3 to 4 minutes on each side. Arrange the skewers on a platter and serve the butter alongside.
If using wooden skewers, soak them in water for 30 minutes prior to grilling to prevent them from burning.