What is the independent variable in this experiment?

I will be testing which factors affect mold growth the most: temperature, moisture, or nutrients.

I am at an understanding that one should only have one or maybe two independent variables in a normal controlled experiment.What is the independent variable in this experiment?
In order to really test which ONE of those factors affect growth the most, you would have to run three different versions, one for each of those variables. In each trial, the other two factors would have to be controlled, so that you could really tell how much your independent variable affects the growth rate, which is the dependent variable.What is the independent variable in this experiment?
I think it's temp!