It seems the only option for printcd3 to ec3 conversion is to use Epson Print CD bundled together with some Epson printer and also available from Epson website. It supposedly supports both .printcd2 and .ec3 files and likely can somehow edit or convert them.
Convert Epson Print CD file to Epson Print CD project file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert printcd2 to ec3 on Windows
A program for creating labels for CD/DVDs
Convert from printcd2 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.