If Pink Floyd only released 3 good albums that's 3 more than most bands! And although i prefer Zeppelin 'Dark Side Of The Moon' is better than any of their records and 'Wish You Were Here' is possibly as good too. I agree that the rest of their records are nowhere near the same quality but The Beatles, Rolling Stones, and other great bands are no different in that respect.
Also 'Echoes' off 'Meddle' is arguably their greatest work!