Creating a Tryptych
The first two videos show methods of creating a trytych from a single image. Something not mentioned in the demonstrations is that to display a grid in both applications used (and in most other software) go to the View menu.
From a single image using Paint:
Creating a Trytych using Libre Office Impress
From 3 images using Affinity
Note - on reflection it would have been simpler to create a blank canvas of the corect size and then brought in the 3 images (in the way that I did after the Flood Fill filled too much!