how to get two text box sum in third text box without button click?

Try this....

<div>
        <asp:textbox id="txt1" runat="server" autopostback="true" xmlns:asp="#unknown">
            ontextchanged="txt1_TextChanged"></asp:textbox>
        <asp:textbox id="txt2" runat="server" autopostback="true" xmlns:asp="#unknown">
            ontextchanged="txt2_TextChanged"></asp:textbox>
        <asp:textbox id="txt3" runat="server" autopostback="true" readonly="true" xmlns:asp="#unknown"></asp:textbox>
    </div>

protected void txt1_TextChanged(object sender, EventArgs e)
    {
        if((!string.IsNullOrEmpty(txt1.Text)) && (!string.IsNullOrEmpty(txt2.Text)))
        {
            txt3.Text = (Convert.ToInt32(txt1.Text) + Convert.ToInt32(txt2.Text)).ToString();
        }
    }
    protected void txt2_TextChanged(object sender, EventArgs e)
    {
        if ((!string.IsNullOrEmpty(txt1.Text)) && (!string.IsNullOrEmpty(txt2.Text)))
        {
            txt3.Text = (Convert.ToInt32(txt1.Text) + Convert.ToInt32(txt2.Text)).ToString();
        }
    }

You can use onTextchanged event of textbox to write the code of adding the values.

<asp:textbox runat="server" id="txtOne" autopostback="true" ontextchanged="txt_TextChanged" xmlns:asp="#unknown" />
<asp:textbox runat="server" id="txtTwo" autopostback="true" ontextchanged="txt_TextChanged" xmlns:asp="#unknown" />
<asp:textbox runat="server" id="txtThree" xmlns:asp="#unknown" />

protected void txt_TextChanged(object sender, EventArgs e)
{
   decimal valueOne = string.IsNullOrEmpty(txtOne.Text) ? 0 : Convert.ToDecimal(txtOne.Text);
   decimal valueTwo = string.IsNullOrEmpty(txtTwo.Text) ? 0 : Convert.ToDecimal(txtTwo.Text);

   txtThree.Text = (valueOne + valueTwo).ToString("N2");
}

txt_Purchase.Text="5000";
txt_Sales.Text="6000";

Now Genrate Txt_Sales textChanged Event(double click on Txt_Sales in design page to genrate event)
and

Write Code in Txt_Sales textChanged Event.
protected void txt_Sales_TextChanged(object sender, EventArgs e)
{
txt_Profit.Text=Convert.ToString(Convert.ToInt32(txt_Purchase.Text)+Convert.ToInt32(txt_Sales.Text));
}

protected void Button1_Click(object sender, EventArgs e)
   {

       TextBox5.Text = (Convert.ToInt64(TextBox3.Text) * Convert.ToInt64(TextBox2.Text) + Convert.ToInt64(TextBox4.Text)).ToString();
       TextBox5.Visible = true;
       Label1.Visible = true;
   }

BY-Arpit Upadhyay Airoli
Please Comment If You Have Question