On a barren island on the outskirts of Abu Dhabi, workers have dug the foundations for three colossal museums: an $800 million Frank Gehry-designed branch of the Guggenheim 12 times the size of its New York flagship; a half-billion-dollar outpost of the Louvre by Jean Nouvel; and a showcase for national history by Foster & Partners, the design for which was unveiled on Thursday. And plans are moving ahead for yet another museum, about maritime history, to be designed by Tadao Ando.