Sum of the Degits on c pgm

#include 

int f1 (int, int, int);

int f2 (int&, int&, int&);

int f3 (int, int&, int);

void main ()

 {

  int i, j, k;

  i=1;

  j=2;

  k=3;

  cout << endl;

  cout << "Initial values of i, j, and k are: " 

    << i << ", " << j << ", and " << k << endl << endl;

  cout << "f1(i,j,k) = " << f1(i,j,k) << endl;

  cout << "Values of i, j, and k after the call to f1 are: " 

    << i << ", " << j << ", and " << k << endl << endl;

  cout << "f2(i,j,k) = " << f2(i,j,k) << endl;

  cout << "Values of i, j, and k after the call to f2 are: " 

    << i << ", " << j << ", and " << k << endl << endl;

  cout << "f3(i,j,k) = " << f3(i,j,k) << endl;

  cout << "Values of i, j, and k after the call to f3 are: " 

    << i << ", " << j << ", and " << k << endl;

}

int f1 (int x, int y, int z)

{

  x=x+5;

  y=y+5;

  z=z+5;

  return(x+y+z);

}

int f2 (int& x, int& y, int& z)

 {

  x=x+5;

  y=y+5;

  z=z+5;

  return(x+y+z);

}

int f3 (int x, int& y, int z) 

{

  x=x+5;

  y=y+5;

  z=z+5;

  return(x+y+z);

}