string[] ids = { "n1", "n2", "n3" };
string xpath = string.Join("|", ids.Select(id => "node[@id='" + id + "']"));

var text = @"<root>
  <node id='n1'>a</node>
  <node id='n2'>b</node>
  <node id='n3'>c</node>
  <node id='n4'>d</node>
  <node id='n5'>e</node>
</root>";

var xml = new XmlDocument();
xml.LoadXml(text);
var root = xml.DocumentElement;

var nodes = root.SelectNodes(xpath);

foreach(XmlNode node in nodes)
    Console.WriteLine(node.InnerText);