Houdini 18.0 features preliminary Python 3 support in the form of a separate set of Houdini build downloads. The Python 3 version of Houdini is a technology preview as described in the CY2019 specification of the VEX Reference Platform. The Python 3 builds are not production ready. They are useful for users and admins interested in getting a headstart in migrating their tools and pipelines from Python 2 to 3.
SideFX plans to release production ready builds some time next year to match the CY2020 VFX Reference Platform specification. Until then, we ourselves will be working to get our Python code updated and upgraded to work with the latest fixes and improvements in Python 3. That means there are currently many limitations. There is currently no support for HQueue or USD, among other issues. Currently, SideFX is not accepting Python 3 bug reports from users. This will change once the number of known issues is reduced and the code is stabilized.
We will continually improve the quality of the Python 3 builds as we publish them alongside the production-ready Python 2 builds. As with the main builds, we will publish bug fixes and enhancements in the online journals so you can track our progress.
To download a Python 3 build, go to the SideFX Daily Builds download page and click the "Houdini Python 3" filter button at the top of the page to view the builds. You can install both Python 3 and Python 2 on the same system as long as you install them in different directories.