So you want some kind of metric to measure how strong a unit is?
Without knowing your map or how your values look like its simply impossible to give a useful answer to that. Is 10000 Hp much in your map? Is lvl 8 close to the maximum level or is the maximum level 9000? Is 250 hp a lot for level 15? Etc..
Give us some examples, like:
1. A hero with 1000 hp, 600 mana and level 20 should receive the same value as a hero with 1400 hp, 750 mana and level 16.
2. A hero with 1000 hp, 700 mana, level 10 should receive twice the value as a hero with 600 hp, 400 mana, lvl 5.
With some of these examples its rather easy to find a spline or polynomial to interpolate these values.
http://en.wikipedia.org/wiki/Interpolation
Also what does this mean:
How shall they increase in an exponential manner? Exponential in every argument (hp, mana, level)? Or exponential in the sum of the arguments? etc... Please provide further specification or write down a formula.