检查CheckBox时,CheckedChanged事件没有触发

它只在我单击按钮(用作清除按钮)时触发,并且我没有在按钮事件中包含该代码！

这是我的代码：

public partial class _Default : System.Web.UI.Page
{
    int total = 0;
    String strtotal;

    protected void ckb1_CheckedChanged(object sender,EventArgs e)
    {            
        if (ckb1.Checked)
        {                
            total = total + 100;
            strtotal = total.ToString();
            lbl2.Text = strtotal;
        } 

    }

    protected void ckb2_CheckedChanged(object sender,EventArgs e)
    {
        if (ckb2.Checked)
        {
            total = total + 80;
            strtotal = total.ToString();
            lbl2.Text = strtotal;
        }
    }

    protected void ckb3_CheckedChanged(object sender,EventArgs e)
    {
        if (ckb3.Checked)
        {
            total = total + 70;
            strtotal = total.ToString();
            lbl2.Text = strtotal;
        }
    }

    protected void Button3_Click(object sender,EventArgs e)
    {
        TextBox1.Text = " ";
        ckb1.Checked = false;
        ckb2.Checked = false;
        ckb3.Checked = false;    
    }

}