Peel the squash and scrub the sweet potato and beetroot. Using a mandolin or food processor finely slice all the vegetables.
Place the vegetables in a bowl and add the olive oil and salt, rub the oil over the slices.
Lay as many slices as you can fit directly onto a baking tray or on parchment paper (do not use baking paper as they stick to that). Ensure that the slices do not overlap. I usually have 3 trays in the oven. The veg shrink in size after about 10 minutes so you can add the rest of the slices when there is space.
Bake in the oven at 100 degrees celsius for up to 3 hours - if you increase the temperature they will cook faster but you'll need to keep an eye that they don't burn.
Once the slices begin to curl, turn them over and continue baking.
Remove from the oven and leave to cool - they crisp up further when cool.