Yeah ive found sm charms very close to 3/20/20's like a 3/20 +5% faster run/walk and other combinations. And yes the +3 to max damage becomes much more than +3 damage - based upon your characters skills. It becomes more and more obvious as you character gets higher in levels/skills.

I'm guessing that the 3/20/20's that are traded so much are dupes though - i can't believe that so many of them have been found legitimately.