My BB8520 came with tons of rubbishy ringtones. I want to delete them all but three. How do I do this?

You cannot delete the built-in ones. Just don't use them.

You might be able to shrink them out of the device OS with BBSAK (or similar software) and then reload the shrunken device OS to the device.

I thought all of the default ringtones were in one file. I could be wrong.

They are. It's a .cod.