I'm doing an average of 3 metrics a, b and c, the lower average the better.
Before I do the average, I want to apply coefficients to these metrics, 'a' being the most important metric, and 'c' the least important.

How can I reinforce 'a''s importance in the final average if the aim is to have the lowest average?

What coefficient should I apply to 'a'? 1.5 or 0.5?