  We present version 3 of the open-source simulator for flow and transport
processes in porous media DuMu$^\text{x}$. DuMu$^\text{x}$ is based on the
modular C++ framework Dune (Distributed and Unified Numerics Environment) and
is developed as a research code with a focus on modularity and reusability. We
describe recent efforts in improving the transparency and efficiency of the
development process and community-building, as well as efforts towards quality
assurance and reproducible research. In addition to a major redesign of many
simulation components in order to facilitate setting up complex simulations in
DuMu$^\text{x}$, version 3 introduces a more consistent abstraction of finite
volume schemes. Finally, the new framework for multi-domain simulations is
described, and three numerical examples demonstrate its flexibility.
