Tiago it's possible to open a Studio One 3 file with Studio One 2 fairly easy.
Here's how: Copy the song file, rename it to [yoursongname].zip and open the compressed folder. Copy out the metainfo.xml and open it with a text editor.
Now look for the following line:
<Attribute id="Document:FormatVersion" value="3"/>
Change the value “3” to “2” like:
<Attribute id="Document:FormatVersion" value="2"/>
Save the XML file, copy it back to the compressed song file. Save and close the modified zip and rename the extension back to [your songname].song.
Now you can open the song again with Studio One version 2.
Take note: Songs can behave strange or unexpected when Studio One 3 features are used in it (like e.g. Scratch Pad, Arrangertrack, Multi Instruments, Channel Editor, NoteFX, Mix FX etc.). Those obviously don’t work in Studio One 2 and most likely are the reason the Studio One developers disabled backward compatibility from version 3. Use at your own risk it's not a supported method!