i3 software package provides metapackage (i3 window manager, screen locker, menu, statusbar), you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install i3
i3 is installed in your system.
Make ensure the i3 package were installed using the commands given below,
$ sudo dpkg-query -l | grep i3 *
You will get with i3 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | X Window System software |
| Package | i3 |
| Version | 4.12-2 |
| Description | metapackage (i3 window manager, screen locker, menu, statusbar) |
| Sub Section | x11 |