MessageBoxDefaultButton.Button3

I wish to know the significant of "MessageBoxDefaultButton.Button3", because without this button following program is giving the same result. Problem is highlighted.

using System;
using System.Windows.Forms;

public class MessageBoxExperiment
{
public static void Main()
{
string message = "This is message ";
string caption = "Message box experiment";
int count = 1;

MessageBox.Show(message + count, caption, MessageBoxButtons.YesNoCancel, MessageBoxIcon.Information,
MessageBoxDefaultButton.Button3);
}
}