To convert a single flac file with a cue sheet to mp3, first split it up:
$ shnsplit -f <sheet.cue> <source.flac>
Then convert the wav-files to mp3 with ffmpeg:
for i in *.wav ; do ffmpeg -i ${i} -codec:a libmp3lame -qscale:a 2 ${i%%wav}mp3 ; done
Or, if you have individual flac files:
for i in *.flac; do ffmpeg -i ${i} -codec:a libmp3lame -qscale:a 2 ${i%%flac}mp3 ; done
qscale:a 2 produces mp3s with an average bitrate around 170-210 kbit/s. It should be close to lossless.