Bonjour Joakim ! ​The expression "le Tiers Monde" to say "the Third World" is fixed as it is in English: this is the only way to say this in French. ​"Le Troisième Monde" would feel like there are more than one world, i.e. one planet, when here it's more of a division of the world into three "parts", hence the use of the fraction here. À bientôt !