Forest Dragon's text reads:

Action: Spend 3 resources to double this unit’s power until the end of the turn.

http://deckbox.org/whi/Forest%20Dragon

There is no limit to the amount of times this may happen. If I have 9 resources can I trigger this action 3 times, doubling its power from 3 to 6, from 6 to 12, from 12 to 24?

Yes.

If you have 2 forest dragons and pay 3 resources.

Do they both get to double their power?

Virgo - thanks!

gertat - No, you have to pay for each action separately

Thanks