With no way to slot extra display cards into a laptop, my first thought was to use a USB-VGA adaptor then a VGA-Composite convertor so I picked one up from Amazon for only £6. Even before purchase it was clear it would be a bit sucky over USB2, but that doesn't matter for a mostly text display.
That's the hard part taken care of and while poking around the Pimoroni site ordering a Zero for the third screen I spotted 8086.net make a USB-UART shim specifically for them and they're cheap. This covers the other half of the job with a readymade board so once the parts show up I'll have three serial terminals connected to the laptop by USB.
There's a big overlap here with my work on creating a library for controlling this kind of terminal from a microcontroller, so coming up with code to drive them won't be new ground. It's a shame they're not full graphical displays but it will make them very easy to spit status info out to. If I need graphics I can go back to investigating the USB-VGA adapters.