Grammar :)
I have heard = present perfect
Used to indicate a link between the present and the past. The time of the action is before now but not specified, and we are often more interested in the result than in the action itself)
I heard = simple past
The simple past tense, sometimes called the preterite, is used to talk about a completed action in a time before now.
I had heard = past perfect
The past perfect refers to a time earlier than before now. It is used to make it clear that one event happened before another in the past. It does not matter which event is mentioned first - the tense makes it clear which one happened first.
More info here:
http://www.ef.com/english-resources/english-grammar/verbs/
*your* time and *your* answers