compare three or more datatables in one class

public DataTable advancesearch1(DataTable ds, DataTable dl)
    {
        DataTable ff = new DataTable("ff");


        ff.Columns.Add("postid", typeof(string));


        for (int i = 0; i < ds.Rows.Count; i++)
        {
            for (int j = 0; j < dl.Rows.Count; j++)
            {

                if (ds.Rows[i]["postid"].ToString() == dl.Rows[j]["postid"].ToString())
                {

                    DataRow drow = ff.NewRow();
                    drow["postid"] = ds.Rows[i]["postid"].ToString();
                  
                    ff.Rows.Add(drow);
                    
                }
            }
        }

        return ff;
       

    }