Make the maple bacon: Preheat the oven to 275 degrees F. Arrange the bacon on a rack set on a rimmed baking sheet. Bake 30 minutes, then brush with the maple syrup and sprinkle with the brown sugar. Continue baking until the sugar melts and the bacon is glazed, 35 to 40 more minutes. Let cool.
Make the candied jalapenos: Combine the jalapenos, granulated sugar and vinegar in a small bowl; set aside.
Make the burgers: Preheat a grill or grill pan over high heat and brush the grates with vegetable oil. Combine the beef chuck, sirloin and brisket, seltzer, Cajun seasoning and a pinch each of salt and pepper in a large bowl. Mix with your hands until just combined. Form into six 1/2-inch-thick patties.
Grill the burgers 3 1/2 minutes, then flip and top each with a slice of Swiss cheese, mozzarella and provolone. Cover and cook 2 1/2 more minutes for medium. Meanwhile, butter the cut sides of the buns and grill until warm, about 1 minute.
Serve the burgers on the toasted buns; top with the maple bacon, candied jalapenos, lettuce, tomato and avocado.