The arraignment is a hearing in court in which the defendant is formally charged with an offense, given a copy of the complaint, and informed of his/her constitutional rights. The defendant enters a plea of guilty or not guilty at this hearing.
The preliminary hearing is not a trial. It is a hearing in court at which witnesses testify and the judge decides if there is enough evidence to require the defendant to stand trial. The jury is not present; the judge alone makes the decision.
A pre-trial conference is an opportunity for the parties to discuss the case with the judge, including possible dispositions and dates for future court hearings.