Yes. The Guide Pédagogique (Teacher's Book) contains the answer key. You can purchase this separately. Some student editions have a Corrigés section at the very back, but Hachette typically removes it to encourage purchase of the teacher's edition.

Don't assign the whole unit. Use a PDF editor (Adobe Acrobat, Preview) to extract 2-3 pages per lesson. Example: Le Mag 3 Unité 2, page 18-19 (grammar practice: le futur simple).

If you are looking for the Le Mag' 3 Cahier d’exercices (Workbook)