public static void Main(string[] args)
{
XElement root = XElement.Load("[your file path here]");
XName[] names = new XName[] { "firstChild", "secondChild", "thirdChild" };
IEnumerable <XElement> elements =
names.AsParallel()
.Select(
name =>
new XElement(
$"result_{name}",
root.Descendants(name)
.AsParallel()
.Select(
x => new XElement(name, x.Attributes()))))
.ToArray();
}