rune of sup absorb

for warrior, is this global meaning minus 3 physical dmg received anywhere on the body at all times or is it just the 1 piece?

The effect is global.

It's global -3 physical damage. Doesn't matter what armor slot you put it in (I'd advise against the helm slot..) you will get -3 on all physical damage you take, no matter where you get hit.

never messed with the absorb ruins can you apply it to each piece of armor for -3 damage each time or does it not stack

It does not stack.

To be a bit more specific - it doesn't stack with other Runes of Absorption. For example, if you put a Superior Absorption on one piece and Major Absorption on another, you will only get the -3 from the superior.
However, it does stack with other mods such as the Knight's Insignia. With a Knight's plus a Superior Absorption you would get -6 damage on the area/piece that the Knight's Insignia is on.

As above, plus this: 3 damage absorbtion, for physical damage only? Survivor/+armor would probably yield much much more survivability.

Yes, Knights and Absorption runes reduce physical damage only. The usefulness of this varies depending on the area.