Whilst both .gp5 and .gp3 files originate from Guitar Pro program, it it doesn't seem possibly to just resave one to another using the program or any other similar music software. That is because Guitar Pro formats lacks backward compatibility.
Convert Guitar Pro 5 project to Guitar Pro 3 project.
We found 2 software records in the database related to .gp5 to .gp3 file format conversion.
Microsoft Windows software - convert gp5 to gp3 on Windows
Multi-track editor and music composer for guitars
Convert from gp5 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned gp5 to gp3. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Linux/Unix software - convert gp5 to gp3 on Linux
Open source tablature editor and player