VOA, any combination that yields the same ratio will perform the same. In your examples of 3:1 gearing (8/24, 9/27, 12/36), there will be no performance difference, although 12/36 might give you a smoother mesh if you can cram a 36 tooth gear in there!
Tall gearing is the term given to lower numeric ratios. IE, 2:1 is taller than 3:1.
Tall gearing, in theory, gives you great top end but less acceleration. Short gearing does the opposite.
When changing ratios, don't make massive changes unless you are certain you need it. Smaller changes (from 3:1 to 3.2:1, for example, will often change performance very noticeably).
If you need more low end power, go to a higher numerical ratio. If you need more top end, go to a lower numerical ratio. Only the ratio matters, not the actual number of teeth on each gear.
Yes, tire diameter (and therefore circumference) is in fact another element in the overall ratio of a drive system, and changing tire diameters DOES change the effective gear ratio. This is a very convenient way to make fine adjustments in drive ratio, assuming that you can afford to make the resulting change in ride height! This is often not feasible with mag cars because you will also be altering the downforce level.