You can see what order they attack by moving mouse over and seeing the number in the middle of the info box. Always use it on the 2nd/last dragon to attack, so that it heals the earlier damage. There are times where you play it when there is only 1 dragon and thus the heal gets wasted at times, but it only gets wasted if you're at full health anyways so its not that big of deal. The only difference it makes is that it wont heal you for EM after all the others attack.