An A3 blank page sketchbook can be an excellent tool for organising anything that informs your legal research: typed notes, handwritten notes, images of book covers, sketches, photos of locations or workshops or experiments, ticket stubs, event programmes. Treat it as a living document in which you can capture multiple layers of snapshots of your thinking in time, and which you can use to explain your thinking to others.
Some examples of my own sketchbooks are below. My process is to type up notes, print them out, cut them up and then stick them in a sketchbook interwoven with other material. I often go back and annotate them by hand soon afterwards to note connections. When I later come to write up my research I use stickers (sometimes in coded colours) to indicate important themes or ideas.
For an insight into how design students use sketchbooks see this video