So your problem is you have a laptop with an inbuilt GPU that doesn’t support more than 3 monitors. One solution is a bigger monitor - that laptop will support 4k monitors, and you just have 3 1080p monitors. A 4k is 4 times the pixels, and if you had three 4k monitors, you’d have 4 times the pixel area you have now. That 40" is especially low resolution for it’s size - I’d just upgrade it to 4k and call it good.
Most windows apps you can split to half a screen with windows key + dragging to the side or arrow key. But you need a high enough resolution for this to be worth it - 1080 is not very many pixels that you can afford to lose half, while 4k generally is.
Solution 2 is to stop using a laptop and use a compact desktop. This is what I do.
Solution 3 is some of those USB to HDMI adapters are weaksauce additional GPUs. You do get another monitor supported because the dongle itself has a terrible GPU in it. Problem is, the latency and limited bandwidth means it won’t be great, but if you use the monitor as your least important monitor that you don’t try to smoothly scroll on, it might be ok. Like this : https://www.amazon.com/StarTech-External-Monitor-Graphics-USB32HDE/dp/B00612ZPQA
That’s got a GPU in it, and with USB 3.0 it might not be terrible.
The NUC Skull Canyon is a laptop, for the purposes of this discussion. Its motherboard design and expansion capability are basically the same as a laptop.
In addition to the options laid out by SamuelA, there’s a 4th option: a Thunderbolt-3 external GPU such as the Razer Core or the AKiTiO Node Thunderbolt 3 eGPU. The latter is $250, but that’s just the enclosure - you need to add a graphics card to it. And it’s a fairly bulky enclosure, so if you bought the NUC because you needed something small and compact, it may not be an option.