In what circumstances should I use a 3 dimensions chart? How many variables may I use, and of what type?

Question

Is it correct to use, for instance, categorical variable for x (doctor name), frequency for y (number of patients), and another categorical variable for z (participant/not participant)?

Answer 1

Generally, it depends on your audience.
In professional circles, 3D graphs tend to be a little juvenile, like using overly fancy fonts.
The simpler you present your information, the easier it will be for others to comprehend it - which is the point.
So for variables, 3 or less is best. Otherwise, the chart gets too messy and confusing. If you want 4 variables, make 2 charts.
Oh, and don't get too exotic with the colors or other touches either.