My understanding is that Studio One 3 (or 3.5) changed the way it handles .song files. So S2 files will open in S3, but once you've saved them in S3, you won't be able to open in them in 2.
The solution is easy: open the project in S3 and immidiately save the project to a new folder - it's good to put an indicator on the folder like 2017-12-17-newproject_s3 or something like that to say that's your version for working in S3.
Saving the project in a few folder for S3 means you have two copies working, and you can always revert to the S2 version.
I've seen some people have the artist version of S3 and the pro version of s3 and they want to edit in S3 and then finish and mix in S2. It won't work that way. When you move a song or project to S3, it's tied to S3 permanently.