Where can I find documentation on how to install falcon 3.4 on php7.3?

I'm not 100% certain, but I think you just need to specify the package. This page gives you directions on how to add packagecloud.io as a repository source: https://phalcon.io/en-us/download/linux

Then, rather than calling the install command in the documentation, you specify what package:

sudo apt-get install php7.3-phalcon=3.4.5-1+php7.3

^ That's according to https://packagecloud.io/phalcon/stable/packages/debian/buster/php7.3-phalcon_3.4.5-1+php7.3_amd64.deb

Sorting through the pages on that site, it looks like 3.4.5 packages start to be listed here: https://packagecloud.io/phalcon/stable?page=50

Good luck.