<first>
    <second>
         <third>
                one, two, three
         </third>
         <third>
                four, five, six
         </third>
    </second>
    <second>
          <third>
                seven, eight, nine
          </third>
          <third>
                one, eight, two
          </third>
    </second>
</first>

    
<second>
         <third>
                one, two, three
         </third>
         <third>
                four, five, six
         </third>
</second>

<first>
    <second>
         <third>
                four, five, six
         </third>
    </second>
</first>

public class JavaApplication3 {

        public static void main(String[] args) throws Exception {
        SAXBuilder builder = new SAXBuilder();
        Document doc = builder.build(new File("f.xml"));
        String a = "one";
        List list = XPath.selectNodes(doc, ".//second/third"); //без переменной
        System.out.println(list);
    }
}

[[Element: <third/>], [Element: <third/>], [Element: <third/>], [Element: <third/>]]

public class JavaApplication3 {

        public static void main(String[] args) throws Exception {
        SAXBuilder builder = new SAXBuilder();
        Document doc = builder.build(new File("f.xml"));
        String a = "one";
        List list = XPath.selectNodes(doc, "//second/third/text()='"+a+"'"); // подставил переменную
        System.out.println(list);
    }
}

[false]

List list = XPath.selectNodes(doc, ".//second/third/child::text()");