[whswhs]

My old ecology textbook had a triangular diagram of climates, with the extremes being glaciated, dry hot desert, and rain forest. You could generate that with two variables, if you allow for the fact that both latitude and altitude influence temperature. But seasonality is important; dry summer climates like California, Greece, or parts of Chile are quite different from wet summer climates with the same total rainfall and comparable temperature. (On the other hand, I'm not sure you do get both the same; east coast places at the same latitude as San Diego get a lot more rain, don't they?) So your three variables are a good start.

The second level, though, is that you want to look at wind patterns. For example, do you have a planet with three wind circulation zones, like Earth? Or one with one, or five, or even some huge number like Jupiter? (This is fantasy; you can have unrealistic planetary physics!) And you want to take into account the rain shadows of mountains.

But I know little about programming, and even less about software. I did my last world design by eyeballing.

What I want is a program that will let me define tectonic plates and project terrains.