How to add extra values in one attribute?

I want to add so many values in one attribute. Below is my code,

        XmlDocument doc = new XmlDocument();
        XmlElement root = doc.CreateElement("root");
        doc.AppendChild(root);
        XmlElement mainelement;
        mainelement = doc.CreateElement("main_element");
        root.AppendChild(mainelement);
        string[] array = new string[] { "one", "two", "three" };
        for (int i = 0; i < 3; i++)
        {
            XmlElement subelement = doc.CreateElement("shop");
            subelement.SetAttribute("Name", "");
            subelement.SetAttribute("client", array[i]);
            mainelement.AppendChild(subelement);
        }
         doc.Save(@"C:\simple.xml");

It gives output like,

<root>
  <main_element>
    <shop Name="" client="one" />
    <shop Name="" client="two" />
    <shop Name="" client="three" />
  </main_element>
</root>

but my expected output is

<root>
  <main_element>
    <shop Name="" client="one,two,three" />
  </main_element>
</root>

help me for do changes like this. Thanks in advance.