Question: how to transfer mp3 files to iphone x?
Answer: Connect your iPhone 6s/7/8/X/XR/XS/XS Max to computer with USB cable and run iTunes.
Locate and click on your device. ... 
To transfer MP3 from PC to iPhone, you need import MP3 files to iTunes library by choosing “File” > “Add File to Library”.
Click “Music” from “Settings” tab.