In a 12-inch skillet heat butter over medium heat until melted. Add onion; cook about 15 minutes or until very tender and golden, stirring occasionally. Stir in maple syrup, vinegar, brown sugar, snipped sage, tomato paste, salt, and pepper. Add navy beans, butter beans, and garbanzo beans; stir to coat.
Cook, covered, over medium heat for 10 to 15 minutes or until heated through, stirring occasionally. Transfer to a serving bowl. If desired, top with sour cream and garnish with tomatoes and sage leaves.