I've been going thru a few boxes of the Remington 3" Magnum 00 buck myself, lately. I've found that in my gun (870 TALO) they pattern a little larger than the 2 3/4" 00 - 26 x 32 at 25 yards - as would be expected. And they carry a noticeablely larger impact: when fired into targets mounted on a standing wood pallet from the same distances I patterned the 2 3/4" rounds, the 3" shells actually knock the pallet over 75% of the time from the 10, 15 and 20 yard distances. They definitely carry a big thump.
The only drawbacks are, of course, that big thump is equally applied to my right shoulder and contributes to the ringing in my ears, despite hearing protection. When running tactical drills using full loads rather than birdshot/target loads I usually only wear ear plugs, since I'm not firing many rounds off. With the 3" mags I absolutely have to wear the cups, as well. I am concerned that in an HD situation I will be so shocked by the noise and flash that I will do something stupid in the fight. I'll not be buying any more of them as the 2 3/4" shells do everything I need from 00 buck. Hope you can find some.