Use iTunes to import the .wav files. Go to preferences and change Import Settings -> Import Using to MP3 Encoder. Select all the .wav files to convert to .mp3 and go to Advanced -> Create mp3 version. Now just select one of the .wav file and go to "File -> Show in Finder", you will find all the converted mp3 files in the same folder.
You have to use iTunes to convert because the command line tool afconvert does not have the ability to convert to mp3.You will get the error Error: ExtAudioFileSetProperty ('cfmt') failed ('fmt?')