The unofficial order of the trilogy is A Fistful of Dollars (1964), For a Few Dollars More (1965) and The Good, the Bad and the Ugly (1966).
I recommend all three of course. Sergio Leone (the director, who also made Once Upon a Time in America also directed what I consider to be the best Western ever made: Once Upon a Time in the West. (Admittedly, I need to see more from the genre, especially those of John Ford.)
Enjoy.
Mick