oldturnip, if you have used the same styles/layers/colours/masters etc. then you can try merging the documents using "Page -> Import…".
Just open the first document, then import the next one creating pages at the end, then the next, etc. etc. until all of the files are collected together.
Once all of the pages are in the same document you can then export to PDF as normal.
However, I've had some problems importing documents where master pages are different - those of the imported document get wiped out - and sometimes text seems to go missing - seemingly at random - so it's not bulletproof. Personally I wouldn't trust this function for anything important, and that's just my opinion, but if your formatting is simple and it will be easy to see any mistakes then give it a try and see what happens.
P.S. It's just a suggestion, but it might have been better to originally ask "How can I merge Scribus documents?", rather than focusing on the PDF/X-3 angle. I'm not complaining, but you might have received different advice from the start.