I have three form in C#, main form is A, second is B & third is C. I wan to close B&C both form (if B&C are actived) from main form using a button.

Assuming that your Main Form code has created instances of Forms B, and C like this:

B FormBInstance = new B();
C FormCInstance = new C();

And the Forms have been shown like this:

private void MainForm_Load(object sender, EventArgs e)
{
    FormBInstance.Show();
    FormCInstance.Show();
}

Then the Button Click EventHandler might look like this:

private void CloseBC_Click(object sender, EventArgs e)
{
    if (FormBInstance != null) FormBInstance.Close();
    if (FormCInstance != null) FormCInstance.Close();
    CloseBC.Enabled = false;
}

I think it's a good idea that if some user-interface Control no longer "does" anything it should be disabled ... or hidden.

But, do keep in mind that once you "close" any instance of a Form: all data, all the results of user interactions with the Controls on the Form, are: lost.