Better way to check the last item of the same type in the if statement

I've written the code that reads .xml document. This code works pretty well but I want to consider other possible improvements.

The structure of the document is:

<rootElement>
     <firstType>someValue</firstType>
     <firstType>someValue</firstType>
     <firstType>someValue</firstType>
     <firstType>someValue</firstType>
     ...
     <firstType>someValue</firstType>
     <secondType>someValue</firstType>
     <thirdType>someValue</firstType>
</rootElement>

So, there are lots of elements of the 'firstType' followed by two respective elements of 'secondType' and 'thirdType'. I need to check if the the current XElement of the foreach iteration is the last 'firstType' element. If the statement is true, do some operations:

foreach (XElement first in hisDoc.Root.XPathSelectElements("firstType")) {
     // some operations
     if ((first.NextNode == hisDoc.Root.XPathSelectElement("secondType")) || (hour.NextNode == hisDoc.Root.XPathSelectElement("thirdType"))){
          // some operations
     }
}

I've coped with the task, though it might be enhanced somehow. I mean if there have been added more other elements e.g. 'fourthType'. It would inconvenient to make longer if statement. I use LINQtoXML(XDocument) and don't wanna change to XmlDocument since it's slower. Moreover XDocument is much more easier to use.

I think it must be more efficient way to do that task.