Download your MODX3 builds right here. There are two versions:
1. Regular installer: download latest build
This is the usual installer you are used to. It's about ~30MB in size. Just download and install by going to /setup
2. Installer with composer.json: download latest build
This is the installer without third-party dependencies. It's about ~20MB in size. Run "composer install" in the root folder first and then run /setup