The future perfect continuous, will + have + been + present participle, emphasises that an action or a repeating series of actions started before a time in the future and is still happening at that time in the future.

http://www.bbc.co.uk/learningenglish/english/course/intermediate/unit-30/session-2/activity-3

The past perfect continuous, had + been + present participle, describes an action which started in the past and continued up to another point in the past.

http://www.bbc.co.uk/learningenglish/english/course/intermediate/unit-14/session-2/activity-3

The present perfect continuous, have + been + present participle, is used to describe an activity that started in the past and is continuing now or has recently finished.

http://www.bbc.co.uk/learningenglish/english/course/intermediate/unit-2/session-2/activity-3