Try this Code:-
protected void Button1_Click(object sender, EventArgs e)
{
this.Button2.Click +=new EventHandler(Button2_Click);
this.Button3.Click +=new EventHandler(Button3_Click);
MessageBox.Show("Buton 2 and Button3 are clicked", "Button1",MessageBoxButtons.OK,MessageBoxIcon.Information);
}
protected void Button2_Click(object sender, EventArgs e)
{
}
protected void Button3_Click(object sender, EventArgs e)
{
}