ROXID Three builds on top of ROXID One. You need to have the ROXID One repository checked out in a directory named roxid in the parent directory of your shop, e.g.:
www/
roxid/
your_roxid3_shop/
ROXID requires a few additional database entries. The easiest way to add those for the development version is building a productive version, and run the sql/install.sql.
Building ROXID Three is analogous to building ROXID One. Run build/build.sh. Please refer to the install instructions in the ROXID One repository.