What pistol are you using? Unless its a SS which will hurt in anything but HM, You can use less pistol mags on the belt. For the rifle, 1 extra should be fine just get an extended mag and never reload on the stage unless its a failure. For shotgun shells, they make an 8 round that is like a double stack 4 which takes the same space as a 4 shell. I usually run 20 round mags for 3 gun and usually carry 1 extra so a max of 2. Shell holders installed as needed. For the rifle I have an extended mag that is a 45 rounder so have never had to reload for the stage. Extra mag is shoved in the back pocket just as an emergency mag if i have a jam that takes the mag in the gun out of operation which I have not had yet.
You just don't have the space on the belt to hold a ton of extra so thinning it out is the way to go.