Put 3 cups water and the mushrooms in a nonreactive saucepan and bring to a simmer. Turn the heat to low, cover, and cook at a very gentle simmer for 10 minutes. Add the kombu and simmer very gently for 5 minutes. Remove from the heat and add the bonito flakes. Let steep, covered, for 8 minutes. Strain through a fine-mesh sieve. Discard the solids and use the broth as you would stock.
