The F3 Plus features a microSD memory card slot. If your personal computer has a slot for that type of card (Most notebooks are so-equipped), you should be able to save music files
The F3 Plus is a smartphone. Most smart phones let you save any mp3 song available online by touching and holding your finger on it, then selecting 'save.' Once the music file is stored on your library, tap-and-hold it and select 'assign' in the contextual menu.
The Oppo F3 Plus is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, Zedge and more.
Here are links to some of the free ringtones (Oppo F3 Plus-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones