There's a name for that type of DR. It's DR 3/piercing or slashing. DR is written as (number)/(what ignores it), not (number)/- vs (what it blocks) in all WotC material I've seen.
Thanks Douglas, had a mental fart I guess. I also forgot to put in that it can only be taken once so it wouldn't stack infinitely. (I'll edit that in.)

From normal to Greater

DR 1/- or ignore 3 points of non-lethal damage
DR 2/- or ignore 7 points of non-lethal damage
DR 3/- or ignore 10 points of non-lethal damage.
Making it a progression would probably be a better idea than just a single feat, thanks for the idea C.C..

'Shrug it Off' and 'Take a Punch' aren't exactly exciting names. Not that I've been doing much better at that aspect of it which is why I'm begging for help. I'm hoping for something more along the lines of 'Vorpal Adamantine Death Skin Power!' which would make my players say, "Cool, I want that feat!"

. . . actually kind of like that name ;)