Ahh, I see what you mean.
What you
can do to achieve something very similar (at least with regard to your Slack window) and which
almost does the same thing is that you can specify a window (or alternatively, all windows for an app) that you want to show on all desktops. I also have a three-monitor setup and use one monitor for the set of applications I always want to see, no matter which virtual desktop I'm on (in my case: Teams, Outlook, Spotify, and a few others that are specific to my work). To do this, you can go into the "Virtual Desktop Switch" mode (Win-Tab) and right-click a window and select the appropriate option.
View attachment 2465735
Note, if you select the "Show windows from this app on all desktops" option, that will stay marked during future Windows sessions until you unmark it again. With that selected, you can also still select individual windows to only show on a specific desktop (handy for something like Outlook where I might want the main app open on all desktops, but an e-mail related to a task that is on one specific desktop can be kept just with that desktop).