It is self-referring(sp?) so yes it is only talking about it self, not all charizards. Infact all card that say for example: "Once per turn you can attach an energy from your hand to Electivire" have this rule applied to them. If it intends for it to affect all of the charizards it would say something along the lines of " If your opponent has three or less prize cards remaining all your charizard's attacks do 50 more damage". Note: that electivire was just an example its not a real card.