Used Air Drop to move a folder with mp3 files in it to my phone. Where are they now? I moved some mp3s to my iPhone using AirDrop expecting they would be available in the Music app, but I can't seem to find them. Where can I find them on my phone?

There was no error or anything saying the files didn't go through. It appears as if they have been copied to my phone.
You can add a 3rd party app that supports mp3 playback to the iOS device and if they support AirDrop you will get a prompt after AirDrop of mp3 from computer to iOS device asking which program to open the mp3.  My prompts include iMovie, Evernote and Google Drive.  I use Evernote.  This does not put it in iOS iTunes.  But I'm not always near the computer that hosts my main iTunes and I use Evernote for the immediate need and then add to iTunes from Evernote on the iTunes main host when the opportunity arises.