As Richard has said, that code does not generate a compilation error, not does it calculate an average properly.
I'd suggest that you start using string interpolation though, it makes your code easier to read:
Console.WriteLine($"The average of {num1 + num2 + num3} is {avgRes}");
And produces the same result.
But don't ever use Convert methods on user input: they fail if he miskeys. Instead, use the TryParse methods:
if (!int.TryParse(Console.ReadLine(), out num1))
{
... report input problem to the user ...
return;
}
With only three inputs it's not a major problem - but if you've just typed 20 values in and miskey the last one ... if the app crashes the user is not going to be a happy bunny ... Write yourself a method to get an integer and loop until the user enters a valid number.