I was confused about that as well when I first started (about a month ago). The gotcha is fava support for beancount version 3 is still in progress. The current version of fava (version 1.29) supports beancount version 2. If you install fava via pip it will install beancount along side fava. Beancount version 2 includes bean-price as part of the installation, no additional repositories needed.
Here are the commands from my notes, that I ran to get started on a fresh Debian container:
apt-get update
apt-get install python3-pip
pip3 install fava
export PATH=$PATH:/usr/local/bin
Run Fava:
fava -H 0.0.0.0 example.beancount