There is difference between "has suffered" (how many damage tokens he has on his sheet) and "suffers / suffered" (how many damage tokens were added by the attack). See Damage from the RRG.
you know, I've always played the right way, checking after every attack targetting him and then if he suffered 3 or more damage in this specific attack he would trigger Rage. However I was double checking the rules for a new campaign I'm starting today and that section on damage in RRG got me confused.
I started to think that after Gaarkhan had 3 damage in his hero sheet, he would always trigger Rage after each attack targetting him.
It is a good thing that you are already clarifying this issue. Thank you very much.