If I have Ancestor Dragon out and attack with 3 creatures does it trigger for creatures attacking and I gain 3 life or does it trigger for each attacking creature and I'd gain 3 life for each of the 3 attacking creatures thus gaining 9 life?
Ancestor Dragon says:
Whenever one or more creatures you control attack, you gain 1 life for each attacking creature.
It triggers only once for each attack if you attack with one or more creatures. If you attack with 3 creatures you gain 3 life.
To clarify, the ability doesn't trigger for each attacking creature, just for each time you attack.
If you attack the second time (for example if you have an additional combat phase) then the ability will trigger again.