Need help finding a way to add a third function to my program

[keskiverto]

Note that you don't use finalSalary anywhere and you don't actually need array for raise.

What if your main would look like:

int main () {
  constexpr int N = 10;
  double currentSal[N] {};
  getsalaries( currentSal, N );
  double final[N] {};
  computenew( currentSal, final, N);
  showsalaries( final, N );
  return 0;
}

That has already more functions than you want, so there is room to cut back.