How to get the last 3 items from an xml file with simpleXml

Question

A bit of a noob when it come s to php and xml but how to get the latest 3 items from a xml file?

$xmlstr= '<?xml version="1.0"?>
<doc>
<tran>
<balance>25000</balance>
<amount>560</amount>
</tran>

<tran>
<amount>5999</amount>
<balance>30999</balance>
</tran>
<tran>
<amount>5000</amount>
<balance>20000</balance>
</tran>
<tran>
<amount>8923</amount>
<balance>25000</balance>
</tran>
...
...
</doc>';