[rragegfffa]

str.replace(/(.*?,){3}/g, '$&\n')

А так можно обобщить:

const adduceEvery = (substr, every, replacer) => 
    (str) => str.replace(new RegExp(`(.*?${substr}){${every}}`, 'g'), `$&${replacer}`)

const commaAdducer$3 = adduceEvery(',', 3, '\n')

commaAdducer$3('world,1,5,591125,25,12,44,41')