The Le S3 is a smartphone. Most smartphones will let you save any song found online by tapping and holding your finger on it, then selecting 'save.' Once a music file is saved to your media library, tap-and-hold it and select 'assign' from the drop-down menu.
The LeEco Le S3 is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
The LeEco Le S3 is Bluetooth-compatible. Bluetooth is a wireless system which lets you connect your cell phone to computers, headsets, car kits, etc. If your computer is also Bluetooth-enabled, simply establish a connection between both devices and transfer your music files from your computer to your LeEco Le S3.
More on free ringtones
Here are some of the 100% free ringtones (LeEco Le S3-compatible) offered here at MartinsRingtones.com:
Free 'Unclassified' Ringtones Free Ghost Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones