I served on a jury three years ago. Do I have to serve again already?

Yes.  Jurors who appeared for service but were not chosen for a trial are exempt from serving again for a period of time determined by the jury commissioner, generally ranging from 1 to 2 years. Jurors who served on a jury panel for a trial in Arizona are exempt from serving again for 2 years. If it has been more than 2 years since serving as a juror, citizens are required to appear for jury service again, regardless of how many times they have served in the past.
Resources:
•    Arizona Supreme Court Jury Service Information webpage