It looks like you got good answers from the tactical perspective. For 3-gun competition, the war belt kind of rig is always going to be a compromise. Most of us competitors run competition equipment - I just started using the Safariland ELS which is a great system for 3-gun, but maybe not so much for your tactical needs.
To answer your questions, 3 pistol and 3 rifle mags should more than suffice, assuming you can hit what you are shooting at (3 x 30 round mags can go really fast if you are missing a lot). For 3-gun I normally have 4 pistol mags so I don't have to carry my loading mag in a pocket etc., and this is way more than I will ever need (68+1 rounds). Usually I carry only one spare 43-round rifle magazine on my belt, with the option of a second rifle magazine around the back if I add an extra pouch via the ELS clip back there. My primary rifle ammo starts in/on the gun, either a 60 round magazine (2 x PMAG w/CCW coupler, or a Surefire) or a pair of cinched 43 round magazines... with this approach (60-86 rounds on the gun) I don't expect to go to the belt for rifle ammo at all unless the wheels have really fallen off. On the rare occasion that I encounter a really high rifle round count, I also have the option of adding a Specter 4-mag thigh rig (also works great for Heavy Metal mags), but this mostly stays in my gear box.
You did not mention shotgun ammo carriers. If you are planning to use the dump pouch, you are going to find this a very slow solution. On the other hand, if tactical considerations are more important to you then maybe you don't care about excelling with shotgun.