Preheat a
12-inch cast iron skillet on the stovetop for five minutes over high heat. Meanwhile, wipe any dirt off the mushrooms with a paper towel and, if you like (and your mushrooms are whole), rinse under running water and spin dry. Remove stems from shiitakes and oyster mushrooms and trim a slice of the stem end off the baby bellas. Slice baby bellas and shiitakes about ¼-inch thick and cut oyster mushrooms into small bite-sized pieces.