I would like to have three tabs open, as well as the left sidebar. I want to keep this layout at all times, meaning that if I open a file in a tab and close it, the number of tabs won’t shrink to two.
I remember having a similar case on mobile. What I did was keep a pinned (dummy) file in the pane I wanted to stop disappearing.
I am sure there is another way.
Just activate workspaces in preferences, core extensions. Back to your main window, click workspaces in the left side ribbon and assign a name to your current setup.