This depends on several factors:
<br>
<ol>
<li>Did you copy the MP3 files from your PC onto your phone or download them directly to your device? If so you could loose them with a factory reset.</li>
<li>Did you sync your MP3 files onto your Google music account and then download them to your phone from the app? If so you can simply factory reset and then download them once more.</li>
</ol>Simply put, if your MP3 files are not synced to your Google music account then you will loose them. You can simply transfer them over to a PC and then transfer them back again or while they are there sync them to your account.
<br>
<br> Link to the software needed:
<br>
<a href='
https://support.google.com/googleplaymusic/answer/1075570?hl=en-GB' target='_blank' class='externalLink' rel='nofollow'>
https://support.google.com/googleplaymusic/answer/1075570?hl=en-GB</a>