yet another DR stacking/overlapping question

[Lamoni]

I know this has been asked and clarified in the past, but my google search didn't turn up any good results.

Here is the hypothetical situation...
A Cleric is wearing adamantine full-plate (DR 3/-)
casts righteous might on himself (DR 15/evil)
and casts stoneskin on himself with the earth domain (DR 10/adamantine)

First of all, I know that they do not stack so a +5 greatsword that does 30 damage would not be reduced to only 2 damage. (option 1)

But, do you have to choose which one to use at the time of time of casting? So you could choose to have EITHER DR 3/-, DR 15/evil, OR DR 10/adamantine? (option 2)

Or, do you have all those at the same time, but only apply whatever is best on each attack? So if you were being attacked by 3 people, one with a magical weapon, one with an unholy weapon, and one with an unholy adamantine weapon... the magic weapon would be reduced by 15 (DR 15/evil), the unholy weapon would be reduced by 10 (DR 10/adamantine), and the unholy adamantine weapon would be reduced by 3 (DR 3/-). (option 3)

I had understood it to be option 3, but then I was presented with option 2 and I wasn't sure that that wasn't accurate. Thanks for any help.

 

[Camarath]

I Think this rule quote makes it pretty clear it is option 3.

"If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation. "

 

[Staffan]

Or, do you have all those at the same time, but only apply whatever is best on each attack? So if you were being attacked by 3 people, one with a magical weapon, one with an unholy weapon, and one with an unholy adamantine weapon... the magic weapon would be reduced by 15 (DR 15/evil), the unholy weapon would be reduced by 10 (DR 10/adamantine), and the unholy adamantine weapon would be reduced by 3 (DR 3/-). (option 3)

I had understood it to be option 3, but then I was presented with option 2 and I wasn't sure that that wasn't accurate. Thanks for any help.

The DRs overlap. This means that if the strongest does not kick in, the second strongest tries to do so. So yes, option 3 is correct.