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')