You could use the same song in different projects, by creating one song and assigning the song to multiple projects, but use different custom fields values in each project, by creating three custom fields and assigning each to a project. It just depends on what you want to do and what data you have. Generally speaking, if you want some item to be the same in every project, you should enter it once and share it; if you want it be different in every project, you should enter it separately in each project.
For example, if you play a song in the same key and using the same lyrics in every project, you should enter it once and share it across all projects.
On the other hand, if you have a custom field for Lead Singer and you will have a different lead singer in every project, you should create a different custom field in each project and then enter a different value for that field in each project.
I don't know what you want to accomplish, so I can't say exactly what to do in your case, but hopefully this helps understand what is happening and what approach you want to take.