I first became familiar with this airbrush back in the day when Iwata’s catalog was only a fraction of the size it is today.  It was initially produced as the RG-2.  Since then the body casting has been updated along with much improved cartridge style mechanicals and redesignated as the RG-3.  It is today ... as it was then ... a ROUND pattern airbrush in a spray gun form factor.  It’s basically an Iwata HP-BE2 in a gun form and it was designed for painting larger areas like backgrounds.  The RG-3 provides superior atomization over a larger area with more robust mechanical reliability and better ergonomics over some sort of bolt on gun style conversion.  The magic of the RG-3 is in the head design that features a large pattern needle and nozzle geometry with an adjustable aircap.  Unlike any other airbrush, the aircap can be fine tuned for air flow to maximize atomization and minimize overspray while taking advantage of the large pattern needle and nozzle.  The swivel color cup allows perfect positioning for whatever angle you’re working at and it can hold enough paint so that you won’t find yourself having to stop during that critical stage of your painting.  Most commonly, the RG-3 is sold with a .6mm needle and nozzle set but .4 mm and 1 mm versions are also available.  Depending on the other airbrushes, you may find the .6 mm ideal.  Since I only use the RG-3 for larger coverage, I find the .6 size a bit redundant so I use the 1mm.