What are letters doing with my numbers 2?

Consider a three digit number $$\overline { ABC }$$:

$$\overline { ABC } ={ A }^{ 1 }+{ B }^{ 2 }+{ C }^{ 3 }$$

$$\frac { A+B+C }{ 3 } =B$$

Then what is C - B - A ?

Details and assumptions: $$\overline { ABC }$$ is a three digit number. This does not indicate multiplication.$$\overline {ABC}$$ is positive.

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