Is the intensity of light shone onto a solar panel proportional to its power output?
In theory, a photovoltaic cell (a single cell) produces a current proportional to the intensity of the incident light. This is due to the photovoltaic effect. The voltage shouldn't change, much, and so power is linearly proportional to intensity.

However, in an array, there will be other effects from the interconnected cells. Furthermore, voltage does show some dependence on intensity, and you need to ensure that none of the panel is shaded, since that severely reduces panel voltage. 

Therefore, in your case, I would say that you cannot simply assume that if the intensity is three times greater, that power will be three times greater.

It might be helpful if you describe your application, since you might get answers that detail alternate ways to accomplish your goal.

For example, if I understand you correctly, perhaps you could build a controlled light source using LEDs, and enclose the entire apparatus in an internally mirrored box. You would know the power going to the LEDs, their spectral emissions, and their intensities based on viewing angle and distance. Furthermore, you should be able to get a datasheet for the panel and determine optimal wavelengths and predicatively model the panel's response.