3" shackles are too long for a lift on XJ leafs. They will put a tremendous amount of stress on the stock leaf packs and fatigue them sooner than they are already prone to.
Most people stop at 1.5-2" shackles and then make up more lift with building a bastard pack, a proper set of lifted leaf packs, or shackle relocation brackets.
You can often find used leaf packs on CL or even in the classifieds here on JF.
I can't think of any pros except you can temporarily lift you rear 3" until it sags.
FWIW - what is the price of the shackles?
'00 XJ - Scampy the XJ
'10 Ram 1500 - Scampy's TB