Combine 2 cups hot water and porcini mushrooms in a bowl; cover and let stand 15 minutes. Drain mushrooms in a colander over a bowl, reserving soaking liquid. Coarsely chop mushrooms.
Bring soaking liquid and broth to a simmer in a medium saucepan (do not boil). Keep warm over low heat.
Melt 2 tablespoons butter in a large saucepan over medium-high heat. Add onion to pan; sauté 5 minutes. Add porcini, cremini, and shiitake mushrooms; cook 8 minutes or until tender. Add rice; sauté 1 minute. Stir in sage, salt, and pepper. Stir in 1 cup broth mixture; cook 5 minutes or until liquid is nearly absorbed; stirring constantly. Add remaining broth mixture, 1/2 cup at a time, stirring constantly until each portion of broth mixture is absorbed before adding the next (about 25 minutes total). Remove from heat. Add remaining 1 tablespoon butter and cheese, stirring until cheese melts.
Wine note: Here's a great vegetarian risotto that's perfect with red wine because of all the mushrooms. Try the northern Italian red Barbera called â€œLe Ormeâ€� from Michele Chiarlo. The 2005, with earthy, dried cherry flavors, is $13. —Karen MacNeil