From Guns.com:

3-Gun competition is unique in that you use three different shooting platforms, but just like a lot of other shooting sports it gets you off the shooting line and moving around. “It’s a pretty physical endeavor… if you want to shoot open and you at to race really hard you have to be in shape,” said Jerry Miculek.
It’s not just the physical limits you’ll be pushing, it’s as much a mental game as it is a physical one.