Extension tubes increase magnification by moving the lens farther from the sensor, but they also reduce the effective aperture, so you lose light. A simple way to judge the light loss in practice is to rely on your camera’s meter, which will account for the tubes automatically.

A rough manual approach is to treat the extra extension as increasing the effective f-number because the physical aperture size stays the same while the image distance grows. For example, adding 12mm to a 60mm lens changes the effective aperture from about f/2.8 to about f/3.5. More extension means more magnification and more light loss.

That said, exact calculations are not always straightforward. Effective aperture depends on magnification and lens design, and more complete formulas also involve pupil magnification, a value manufacturers usually do not publish. So precise numbers for every setup can be hard to derive from specs alone.

In short: yes, stacking 12mm + 20mm + 36mm tubes will noticeably increase macro capability, but you will also lose light. If you want exact exposure impact, the most practical answer is to mount the tubes and read the camera meter.