I shot my first real 3 gun match a couple weeks ago. I found that I had a lot of extra mental bandwidth- especially with so many transitions between weapons and positions. It seems like when I'm just driving the pistol at a pistol match I'm much more focused on what's going on- probably because I've been doing it for a couple years-
With 3 gun, I have much less practice, so everything took longer (even if it was some small fraction of a second)- so I fumbled a shotgun reload- and was thinking,"That's ok, just load that one, then get the next one, push it in the gun, nice and easy, don't drop it." Things like that.
I think, for me, that I've spent a fair amount of time practicing with the pistol- to have a proper draw, a proper reload- that when I reload the rifle it takes a second, maybe 2, longer than the pistol- which when you're used to fractions of a second, can be an order of magnitude more time- which is a lot. Like going from 2 seconds to 20... There's a lot of extra time in there.
So I think with the pistol, I execute the plan I made before hand. With 3 gun, I execute, but my pace is so much slower by comparison, that there's a lot of extra time- and everything is on the conscious side, while the pistol is subconscious...