The only utility I know of that has the ability to trim and re-save mp3 files in NATIVE mp3 format is something called "MP3 Trimmer".
This means that the file is NOT
- decompressed before editing into an AIFF or something else
- edited, and then...
- RE-compressed back into mp3 (which means the file goes through "the lossy process" AGAIN, losing more audio info).
MP3 Trimmer (again) worked with the file "natively", no de- or re-compression.
There may now be OTHER utilities that can do this, but I don't know of them.
However, THERE'S A PROBLEM:
MP3 Trimmer IS NOT 64-bit compatible -- meaning it will run ONLY with OS Mojave or earlier.
This excludes all m-series Macs.
But IF you have an Intel-based Mac, and IF you're using Mojave or earlier, it's the best there is.
If anyone knows of a modern app that can work with mp3 files in their native format, please let us know...