Understanding call-by-value and call-by-reference

#include <iostream.h>

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);
}