Be aware you will degrade the MP3’s by re-encoding them.
If you still want to do it, you could try “TrimExtend” ( https://forum.audacityteam.org/t/trimtrack-plug-in/14323/7 ).
See here for installing Nyquist plug-ins http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#install .
Then add TrimExtend to a chain using File > Edit Chains ( http://manual.audacityteam.org/o/man/edit_chains.html ) and File > Apply Chain, choosing “Apply to Files…”.
Gale