Form1中按钮点击事件:
private void button1Click(object sender, EventArgs e)
{
Form2 frm2 = new Form2();
frm2.MdiParent = Form1.ActiveForm;
frm2.Show();
}
Form2中按钮点击事件:
private void button1Click(object sender, EventArgs e)
{
Form3 frm3 = new Form3();
frm3.MdiParent = Form1.ActiveForm;
frm3.Show();
}

就这么简单,你试试吧.呵呵.