Import the MP3 into GarageBand on your iPhone to "Share" it as a Ringtone.
for the purpose of recording or setting as a custom ringtone. How to Manage Notification Sounds in the App facebook pop notification sound download
Apple does not allow direct MP3 downloads for ringtones via the Files app. You must convert the file to M4R using GarageBand (free on all iPhones). Import the MP3 into GarageBand on your iPhone