Expected. Even though they install to different folders, they still use the same preferences file and data store config file. Those files were upgraded as soon as you ran version 3 the first time, and version 2 won't understand them. To go back to using version 2, you'd have to restore those files from the backup that you created before you upgraded to version 3. Which you have, because you took a backup before upgrading to version 3. Like everyone does. Right?
And here's the catch: remember I said that even though version 2 and version 3 install to different folders that they use the same preferences file? If you installed version 3 and ran it, then it upgraded the preferences file. If you then ran version 2, it would not understand the preferences file and would give you an error (which you probably saw, based on your previous post). Since version 2 didn't understand the file, it would [internally] created a new, default set of preferences. Upon exiting version 2, it would have written that new, default set of preferences out to the preferences file. The same one that version 3 uses. Now all of your preferences (for either version) have been lost and replaced with a default set of preferences from version 2.
This is exactly what happened to me, for clarity. So how do I replace those lost default preferences from version 2?
This is referring to someone who had version 2, installed version 3 and tried it out, then decided to continue using version 2 instead of upgrading to version 3. Version 3 understands version 2 files. But version 2 doesn't understand version 3 files. When you have version 2 and then download, install, and run version 3, it will upgrade your preferences, your player database, and any files you load AND save with it. When you go back to using version 2, version 2 won't understand the new format of the preferences file, database, or any of the files that were SAVED with version 3.
There isn't any need to import files from version 2 because version 3 treats version 2 files the same way it treats version 3 files. They should just work. You just load them like any other files.
You can restore a backup from version 2. That should work just fine. But be aware that if the backup contains your database, it will overwrite your existing database. This may or may not be what you want. And from have you lost formulas? This matters because stats formulas are stored in the preferences, and tournament formulas (points for playing) are saved with tournaments. You might want to simply unzip a backup file somewhere (other than the TD's data folder) and load a saved tournament from it...