Monster is モンスター "Monsutaa" , but the raw uses kaibutsu instead, just as it uses sanningumi instead of "Torio". They're synonyms, that one word has one meaning doesn't mean there can not be another word with exactly that same meaning, specially when a language incorporates loanwords into its lexicon.
jisho.org gives trio as a valid translation of sanningumi,
https://jisho.org/search/sanningumi
They get their translation data from JMnedict, as you can check in their about page
https://jisho.org/about
I am sure you can find many other online dictionaries that translate it as trio, such as:
https://en.bab.la/dictionary/japanese-english/三人組
https://www.linguee.com/japanese-english/translation/三人組.html
https://eudict.com/?lang=engjpk&word=trio, gang of three, thr eesome
Of course, you could say Monster Trio is a fanmade term, because the non-fanmade term is kaibutsu sanningumi and everything else are just translations.
If you want to be that strict and literal with your translations, Strawhat Luffy is also a fanmade term, it should be mugiwara no luffy, aka Straw Luffy, since mugiwara just means straw (strawhat is mugiwaraboushi)