you did not actually CALL the function. you must call it!
int num1 = 0;
int num2 = 0;
int num3 = 0;
cout << "Enter the first number: ";
cin >> num1;
cout << "Enter the second number: ";
cin >> num2;
calcSum(num1, num2, num3);
as a side note this is a bit odd.
most would do one of 3 things here.
int calcSum(int a, int b) //calculates and returns, letting main decide to print or use it..
//in main, num3 = calcSum(num1, num2);
or maybe this, removing the somewhat useless 3rd input, printing it here
void calcSum(int a, int b)
cout << a+b<< endl;
and a somewhat strange 3rd option (return in c, main handles result)
void calcSum(int a, int b, int &c)
c = a+b;
Nothing you have is wrong, beyond forgetting to call it.