Three column layout for web interface

### What problem does your feature request solve?

A commonly requested addition to the web interface is a three column layout. This was requested in formal user interviews during my MSc, as well as more casually in conversation will several people.

A three column layout is good for comparisons as it allows two possibilities with a reference. It is commonly used for Control-Experiment-Observations or Control-Experiment-Difference layouts.

### Describe the solution you'd like

Much like we have the two column layout, we should have some way of putting plots into a three column layout.

I'm not sure yet on how to arrange the buttons on the sidebar for putting plots in place. Perhaps we have a toggle that changes the buttons from [double-left, single, double-right] to [triple-left, triple-centre, triple-right]?

Furthermore, with the sidebar visible there isn't that much horizontal space for a three column layout, so we might also want to investigate making the sidebar collapsible.

### Describe alternatives you've considered

In #1713 we implement pop-out plots, which open in their own windows. This can be used to fulfil this need, however it can be cumbersome arranging those windows, so I think this is still desirable to add within the GUI.

I also think that three column is reaching the limits of what you can fit usefully on a single screen, though you could perhaps push 4 quadrants too. So I don't think we need to go for a full Jupyterlab style windowing system.

No files or tests are named. Start by locating the existing two-column plot layout and its sidebar controls, then review the pop-out approach referenced in #1713. The work is done when the project has an agreed three-column interaction and layout, including a decision on sidebar behavior and supported plot placement.

Written by the indexing model from the issue text.