sorry for the late reply. PDF24 Creator can create PDF/X-3 files (other PDF/X versions such as PDF/X-4 are not available):
Open the image in the PDF24 Creator, for example by dragging it into the window (in the PDF24 Toolbox: Open PDF24 Creator). Alternatively print the image on the PDF24 printer, then the PDF24 Assistant opens.
Click Save as... and select the format PDF.
Set PDF standard to PDF/X-3.
On the page PDF/X you can enter a Title and select the Printing condition your print shop requires (or Custom with your own ICC profile). Then save the file.