The workaround I did, was printed the PDF files to PDF (again) using Ubuntu 16.04's default PDF viewer, and then I was able to merge these 3 PDFs.
The PDFs were originally gmails that I printed using Chromium's print to PDF feature. Printing them to PDF again using a different too seems to clean out what ever was corrupting this the first try to merge them.
The workaround I did, was printed the PDF files to PDF (again) using Ubuntu 16.04's default PDF viewer, and then I was able to merge these 3 PDFs.
The PDFs were originally gmails that I printed using Chromium's print to PDF feature. Printing them to PDF again using a different too seems to clean out what ever was corrupting this the first try to merge them.