Before beginning, drain and rinse two of the cans of black beans and set aside. Using a potato masher, mash up the remaining can of beans with the juice. Add the salsa and cumin to the mashed beans and set aside. In your pot, combine the broth, water and onion, and bring to a boil. Simmer till onions are tender. Add the rinsed and mashed beans and stir well. Simmer at a low heat for half an hour or so. Serve with a dollop of sour cream.