3. This means that, if you have this as a faceup damage card, the next time you suffer critical (faceup) damage with Fuel Leak faceup, you draw an additional facedown damage card and flip the Fuel Leak card facedown. Or, you can flip it over as an action.
Just a small clarification, you don't atomaticaly draw an extra damage card if you have this card face up when suffering critical damage. You suffer one extra (non critical) damage. So if you have shields (mabye you have a chirpy astromech that has regenerated a shield for you after receiving the Fuel leak) when that happens, you flip over a shiled tooken instead.