The core activity of atelier3j encompasses three main areas: 1. **Architecture**: The company provides architectural services ranging from planning to the realization of construction projects. 2. **Project Management** (Assistance au maître de l'ouvrage): atelier3j supports building owners in the planning, coordination, and supervision of construction projects. 3. **Photography**: The company also offers photographic services, likely related to the documentation of construction or architecture projects.