Finally got a server I can mess about with MM3 on. Only took a year :(!
Anyway, I have compiled Mailman3, and the web interface is coming up. Bringing up the list of lists initially gave me an exception, and I saw a 5xx error in the mailman.log file in /opt/mailman/var/logs. I read up on this and as a result downgraded Falcon<1.0 in the Python 3.4 venv.
I am still unable to browse the list of lists with a 500 error, only this time I am not getting an entry in the mailman.log indicating there is an issue, and I am wondering if there are more detailed logs that can be switched on?
The other question I have is on Sassc. I believe the default Mailman web interface has now switched to the Ruby implementation of Sass, but is this correct? The documentation on the Mailman-Bundler site seems to suggest that Sassc is still the default.