Transfer mp3 files from macbook air to iPhone 5s?

iOS: How to transfer or sync content to your computer

The first one is using iTunes to sync music from iTunes to iPhone. The second way is trying iPhone music transfer program like FonePaw iOS Transfer. If you prefer not to get the iTunes involved, you can directly use the iPhone music transfer program. Of course, you can also watch the tutorials like Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB - Apple Support.

You can find many other ways on google. But I think they are some similar.