Adjourn is what you say when you end an ongoing meeting (one that you HAVE STARTED) in order to continue it on a later occasion. Postpone and defer are when you decide to basically start it later than originally planned.

Adjourn-meeting was in process and currently stopped and will resume at an established time or date in the future. Postponed/Deferred- meeting/event start times or dates have been rescheduled to a future time or date.

Postpone means arrange for something to take place at a time later than that first scheduled. Adjourn is to put a meeting on pause or to take break until later. Defer is to delay something until a later time. Defer usually doesn't refer to a meeting.