I think you are going to have to either make use of some complex mapping, using lots of functoids, or make use of a scripting functoid to achieve this.
Assumming all the ExtraHeaderNotes elements are present even when empty, you should be able to make use of some logical String funtoids to check the value of each element and then use the output from these, in conjunction with a few Equals functoids, some AND functoids and some Value Mapping functoids to control which of the values gets mapped.
e.g.
If (ExtraHeaderNotes1 is string) and (ExtraheaderNotes2 is not string) and (ExtraheaderNotes3 is not string) map ExtraHeaderNotes1
If (ExtraHeaderNotes2 is string) and (ExtraheaderNotes3 is not string) map ExtraHeaderNotes2
If (ExtraheaderNotes3 is string) map ExtraheaderNotes3
Not that easy to explain in text but I hope you get the idea!