Uploading from iPhone to iTunes Match Library
I know this is not exactly what you are looking for but in case you can't find an exact solution, here is an alternative:

Assuming you are getting the MP3s off of the internet, Safari should automatically save the MP3 file into the Downloads folder on iCloud. You can listen to the MP3 by tapping on it in the Downloads folder under iCloud Drive in the Files app. You can also move the MP3 to another folder under the iCloud Drive after selecting the MP3 if you want to keep the downloaded MP3s together in a separate folder. However, files downloaded as such will not be part of your iTunes / Apple Music library.