How do I say this in french?

If you want to use the future tense, you may write:

Lorsque je terminerai mes (examens) terminaux (if it deals with university exams), je retournerai/reviendrai en Californie pendant 3 semaines.

You may also finish your sentence by "je retourne/reviens.....".

From a grammatical viewpoint, it is absolutely correct.

BUUUUUUT I would avoid this second solution because "je reviens" (and not "je vais revenir" which would be translated by "I am going to go back") implies an action in a very close future.

In the French version, if you say "je reviens" which implies an action in a very close future, it means you precisely or/and implicitely know when you finish your exams.

Après mon examen final je retournerai/reviendrai en Californie pour trois semaines.

You have to use future tense.

You say EN Californie; la Californie is feminine, while "au" is the contraction of "à le" which is masculine. Ex : au Chili, au Kenia, au Maroc, au Texas (le Texas) but en France, en Colombie, en Tunisie, en Californie (la Californie).

revenir means "come back".

Après avoir terminé mon examen final, je vais retourner au Californie pour trois semaines.

à Los Angeles, because it is a city or town, but au Californie because it is a state. But you will be "en Californie" once you are there.

Darius is correct. Both retourner and revenir can be used here.

lettuce = los angeles salade milk = le lait ice cream = los angeles glace bread = le soreness banana = los angeles banane orange = l'orange apple = le pomme carrot = le carrot cake = le gateau egg = l'oeuf cheese = le fromage ear = ? eye = ? red = violet crimson = rose orange = orange yellow = jaune black = noir pen = le stylo pencil = le crayon table = le bureau frog = ? horse = le cheveau get dressed = ? blouse = ? pants = pantalon shawl = ? educate = gare lady = ? woman = ? little one = ? guy = ? Boy = le garcon plate = plat cup = le verre soz if unsuitable spelling

i would use retourner instead of revenir