un costume (trois pièces) : a (three-piece) suit
A costume worn for fun is called un déguisement (literally "a disguise") in French.
Pour son mariage, Paul portait un costume trois pièces et un chapeau.
des costumes (de théâtre) : (theatre) costumes

Now that you know how to translate 'Costume-travail' in English, why not go further and test our online French course for free?
What our users say:
Test your French with Frantastique today and get a free level assessment.
Absolutely free - no strings attached.