IPSC 3 Gun is very different from CAS. The stages are almost always single weapon, and guns are never staged on the range.
The rifles, to be competitive, are generally high capacity semi auto's, mostly military style semi auto's. Most of our member are using .223, 33 round mags are the norm. Some use optics, and some more than one scope at the same time.
The shotguns are generally long tube semi auto gas operated.
The pistols are what ever that person shoots IPCS with, but rifle, pistol and shotgun must be the same "class" ie. if one uses optics all must (limited exception -- some matches allow scoped rifle, iron sights for pistol and shotgun).
Rifle distances out to 300metres, shotgun -- maybe 20 metres, pistol out to 50 metres.
Almost always shot while moving , and remember our score from hitting targets is divided by your time.
Have a look at the photos on the IPSC section of the website, 3Gun subsection.