Pg. 562 (Core) talks about magic weapons of +3 or greater ignoring some types of Damage Reduction. If I understand the chart correctly then a +3 sword can ignore Cold Iron/Silver DR, +4 can ignore these and Adamantine and finally a +5 can add alignment based DR to the list. Correct?
Or perhaps it means if a Creature has 5/Cold Iron DR, then either a Cold Iron weapon or a +3 weapon ignores the Damage Reduction?
I think you're correct, though my group has never really discussed this (I think for fear of our GM shutting it down). If a creature has cold iron DR, then either a cold iron weapon or a +3 magic weapon will do the trick. Same goes for the other things mentioned on the chart.
Had no idea that was the case.