Studio One 3 is not officially supported on Catalina, it will open and run, however only Studio One 4.5.4 is officially supported on Mac OS X 10.15
(at the time of this writing) Many plug-in companies have not yet endorsed 10.15, make sure any plug-ins you're using have 10.15 support.
Studio One 3 (64-Bit) will launch in Mac OS X 10.15. However Catalina does not support 32-Bit at all, if you have any previous Studio One 3 projects that are using 32-Bit plug-ins, this may be why your projects are not loading properly. Make sure you launch Studio One 3 in 64-Bit mode and use only 64-Bit plug-ins, then loading your project may succeed. You may need to go back to a previous Mac OS and open it in 64-Bit Mode, load your plug-ins with 64-Bit, save it and then try reloading it in Catalina.
