How to merge 3 xml file into 1 xml file using finalbuilder7

When I run my unit testing project using nunit 3, the results will be saved as xml files in a specified location. I have web service seperatly, in which I need to pass xml string as input by combining 3 or more xml files.

I am using finalbuilder 7 to run my unit testing. I have checked "Merge XML" file action in final builder. It merges only two files to single xml.

Is there any option to merge 3 or more xml files into single xml file ?