LONDON/FRANKFURT, July 28 (Reuters) - Three banks from the European Union failed to meet binding capital requirements in a stress test that saw a theoretical 496 billion euros ($546 billion) wiped from their buffers, the bloc's banking watchdog said on Friday.Bank stress tests became a feature in Europe and the United States after the 2008 global financial crisis when taxpayers had to bail out some undercapitalised lenders. They are now part of routine supervision to ensure banks can still support the economy even in times of stressed markets.The European Banking...