Kudos to Infininight at Myth Village who has reported the availability of the Myth III solo demo for PC at Myth Guide. The 98 MB demo is available on Myth Guide's demo page and is currently exclusive to Gigex.

At this point, the included level hasn't been specified. Let us know what it is by replying to this post.

UPDATE: Ramses II let us know that the demo includes The Nest and Siege Of Llancarfan levels.