Something like this:
protected void gridview_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
Textbox tb = e.Row.FindControl("tbAddUp");
Textbox tbNum1 = e.Row.FindControl("tbnum1");
Textbox tbNum2 = e.Row.FindControl("tbNum2");
tb.Attributes.Add("onBlur", "MyAddClientSideMethod('" + tbNum1.ClientID + "','"+tbNum2.ClientID+"');");
}
}
In this way all three textboxes belonging to specific row of gridview will be registered with your client side function. So now you already have ClientIds with you in your client side function. This way it will solve your issue.
Hope this helps !!!