Scrub the sweet potatoes. Place them in a large saucepan, cover with water, and simmer until soft (about 20 minutes). When cool enough to handle, peel the sweet potatoes and cut into 2-inch thick pieces. Set aside.
Melt 3 Tablespoons butter or margarine in large skillet over medium heat.
Add bananas and fry, turning often, until well coated and heated through (about 5 minutes).
Add sweet potatoes, toss carefully to coat, and heat through, about 5 minutes.