In English, when we list multiple items, we often use a preposition just once: "I am dreaming of a shampoo, treatment, and a little volume."
In French, the prepositions à, de, and en are strictly repeated. You must place them before every single item in a coordinated list.
Repeating "de"
If your sentence uses a verb or phrase that requires de (like avoir besoin de or parler de), you must repeat it for each noun. This includes its elided form (d') and its contracted forms (du, des).
| What I need is a coloring, a treatment, and a blow-dry. | Ce dont j'ai besoin, c'est d'une coloration, d'un soin et d'un brushing. |
| He is talking about the coloring, the balayage, and the blow-dry. | Il parle de la coloration, du balayage et du brushing. |
Repeating "à"
The same strict rule applies to à (to/at). Remember to use the contracted forms (au, aux) when it combines with the definite articles le or les.
| He is interested in the shampoo, the coloring, and the hairspray. | Il s'intéresse au shampoing, à la coloration et à la laque. |
| He pays attention to the roots, to the locks, and to the tips. | Il fait attention aux racines, aux mèches et aux pointes. |
Repeating "en"
Whether you are talking about someone's field of expertise or the material an object is made of, the preposition en (in/made of/as) must also appear before each noun in the list.
| As a specialist in balayage, in coloring, and in treatments, he is very busy. | En tant que spécialiste en balayage, en coloration et en soins, il est très occupé. |
| The wooden and plastic combs cost more than I thought. | Les peignes en bois et en plastique coûtent plus cher que je ne pensais. |
Direct objects
If a verb takes direct objects and doesn't require a preposition at all, this rule doesn't apply. However, keep in mind that you still must repeat the basic articles (like le, la, or les) before every noun!
| What I hate is the blow-dry, the hairspray, and the volume! | Ce que je déteste, c'est le brushing, la laque et le volume ! |