I Robot, The Caves of Steel, The Naked Sun, The robots of the Dawn, Robots and Empire, The currents of Space, The Stars like Dust, Pebble in the Sky, Prelude to Foundation, Forward The Foundation, Foundation, Foundation and Empire, Second Foundation, Second Foundation, Foundation's Edge, Foundation and Earth.
I guess thats the complete Asimov Foundation Series, there were a bunch of books set in the universe, not written by him that fit in there somewhere but i dunno.
Jafo - Asimov could certainly count - HE never referred to the Foundation stories as a trilogy - Remember - they were written and published in serial form in the magazine 'Astounding', and only put in book form (as a trilogy) by the publisher, much, much later