Hi, yes they do fit. I have Disco 3 18" wheels on my 53 plate Disco 2 facelift..
You need a set of spigot rings that fit over the Disco 2 hub, that make sure the Disco 3 wheels are centralized correctly, these are about £8 a set off ebay, you will also need a set of Discovery 3 wheel nuts, the flange on the Disco 2 nuts is too big for the Disco 3 wheels.