can i use server control in document.form.element.value in element javascript

I have three textbox

<asp:TextBox ID="TextBox1" runat="server" Text="0" ></asp:TextBox>

<asp:TextBox ID="TextBox2" runat="server" Text="0" ></asp:TextBox>

<asp:TextBox ID="TextBox3" runat="server" Text="0" ></asp:TextBox>

And one javascript function

<script type="text/javascript">


function calc()
{
        var _txt1 = document.getElementById('<%= TextBox1.ClientID %>');

        var _txt2 = document.getElementById('<%= TextBox2.ClientID %>');
        var _txt3 = document.getElementById('<%= TextBox3.ClientID %>');
        var t1=0, t2=0;
        
        if(_txt1.value != "") t1=_txt1.value;
        if(_txt2.value != "") t2=_txt2.value;

        _txt3.value = parseFloat(t1) + parseFloat(t2);
        document.BuyForm.TextBox3.value = _txt3.value;
        
       
    }
   
    </script>

But i have difficulty to assigning textbox value like "document.BuyForm.TextBox3.value = _txt3.value;"

how can i use server control in document.form.element.value in element?