Preheat broiler with 1 rack on the bottom of oven and 1 rack about 4 1/2 in. from heat.
In a food processor, whirl garlic cloves until finely chopped. Add butter, granulated garlic, and cayenne and whirl until smooth and well blended, scraping bowl as necessary.
Halve bread lengthwise and evenly coat cut sides with garlic spread. Place bread, cut side up, on a baking pan and broil on bottom rack until butter is sizzling, about 2 minutes.
In a medium bowl, toss together cheddar, jack, and parmesan cheeses.
Remove bread from oven and sprinkle evenly with cheese mixture. Return bread to top oven rack and broil until cheese has melted and is just beginning to turn golden, about 1 1/2 minutes. Transfer bread to a cutting board and cut into 1 1/2-in.-wide slices.
Note: Nutritional analysis is per serving.