Auto Decimal numbers in asp.net textbox

eg: if you enter 300 then it converts to 300.000, 33.331 then 33.331

.aspx :

<div>

   <asp:TextBox ID="txtdramt" runat="server" AutoPostBack="true" ontextchanged="txtdramt_TextChanged"></asp:TextBox>

    </div>

.cs :

protected void txtdramt_TextChanged(object sender, EventArgs e)

    {

        try

        {

            double num = Convert.ToDouble(txtdramt.Text);

            string numfix = num.ToString("N3");//N3 is for three decimals.

            txtdramt.Text = numfix;

        }

        catch

        {

            txtdramt.Text = "Please Enter a Number";

        }

    }