Preheat the oven to 175°F (if you have a convection option on your oven, this works best as it dries out the citrus faster and more evenly).
Line two large baking pans with parchment paper and then place a wire rack on top of each of the parchment paper-lined pans. If you don't have wire racks that fit your pans, you can bake them right on the parchment, but it will take longer.
Slice the 3 navel oranges into thin slices (you can easily use more or less depending on how much you want to make; three regular oranges fill two 12x17-inch baking pans perfectly.) You should get about 12 slices per orange after removing the ends where the slices are mostly pith.
Lay the slices out between layers of paper towels and press to absorb as much of the juice as possible; this will help reduce the baking time.
Arrange the prepared orange slices on the wire racks and bake for 1 hour. Flip the slices and bake for 1 hour. Flip again and bake for 1 more hour for 3 hours total.
Check the slices after the initial 3 hours of baking. Transfer any slices that are fully dried to a plate. Any that sill look plump or that are still a bit sticky can be flipped and go back in the oven for 30 minutes.
Check the slices again, remove the slices that are done, and bake the rest for another 30 minutes. Repeat this process until all of the slices are fully dried. Note that if using a non-convection oven or baking directly on the parchment could add 1 to 2 hours to the baking time.
Use as drink or dessert garnishes or decor.