Modia3D is an experimental Julia package to model and simulate 3D mechanical systems. Ideas from modern game engines are used to achieve a highly flexible setup and features of multi-body algorithms are used to get a rigid mathematical formulation and support, for example, of closed kinematic loops. Collision handling is performed on convex geometries with elastic response calculation. A Modia3D model is solved with a variable-step solver. This is important to combine Modia3D with the equation-based modeling system Modia in the future