Searching the PureOS repo on https://software.pureos.net/categories didn't show any results for i3 i3wm or sway. It's debian so I assume it's possible if I add a repo but I'd prefer to use official.
You can install
i3 on PureOS the same way as on Debian:
sudo apt install i3
Under PureOS "amber", this will install i3-wm with dependencies, such as i3status, i3lock, suckless-tools. After installing, logout, choose i3 session under the gear ⚙ menu, and login.