The error actually says it all:
button3_Click(object, System.EventArgs)' must declare a body because it is not marked abstract, extern, or partial
You probably have a button object named button3 drawn in your UI and you have wired a click event for it.
Anyways, you need to modify the method so that it has a body. So instead of
private void button3_Click(object sender, EventArgs e);
you should have something like
private void button3_Click(object sender, EventArgs e) {
... do something over here...
}
If that method isn't actually needed, either comment it out or remove it completely.