Cut the baguette in half, and then slice each half horizontally.
Preheat a grill pan over medium heat. Spread a half-teaspoon on the outside of each piece of bread. Lay the flat pieces of bread, buttered side down on the hot grill pan.
Layer the Gruyere cheese and Swiss cheeses; sprinkle each half with Parmesan cheese.
Place remaining pieces of bread on top, buttered side up.
Place a cast iron or other heavy skillet over the sandwiches and lightly press down.
Grill sandwiches for 2-3 minutes or until golden, turn sandwiches over and place the skillet back on top.
Heat sandwiches for an additional 2- 3 minutes or until cheese has melted.