What event is triggering the close of the two windows?
This.Close()
will only close the form on which the code is written. That's what the "This" part of it means. So if you have a button on Form3 named btnExecute and you want it to close Form1 and Form2, you would have to do something like this: (it's in vb.net, but you should easily be able to convert it)
Private Sub btnExecute_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExecute.Click
Form1.Close()
Form2.Close()
End Sub
UPDATE - It's really easy to switch from VB.Net to C#....here is the C#:
private void btnExecute_Click(object sender, System.EventArgs e)
{
Form1.Close();
Form2.Close();
}