Best and also cheap test for cytotoxcicity dertemination is MTT test. In this method it is better that remove scaffolding before reading the plates by Elisa reader.
Viability of 3D culture seeded cells is achievable via MTT assay on scaffold/ after removing scaffold. Application of scaffold free procedure may limit restrictions related to Microplate reader noise caused by scattering or turbidity problem.
For 3D culture, the MTT assay is widely used. For 2D culture, flow cytometry is used a lot to verify viability. However, I don't know if it is recommended for 3D culture.