I have a set of shackles for the rear that are about 3inches longer than the stock shackles from my 82, does this mean that it'll give me a 3" lift or is it 1.5"........any help would be great

It'll give you 1.5"

Yes, about 1.5" lift, more or less:
http://www.4crawler.com/4x4/ForSale/Shackles.shtml#FAQ4

Will also add about 3 degrees to your pinion angle that may need to be corrected:
http://www.4crawler.com/4x4/ForSale/Shims.shtml