Ferrex pointed out on RampancyHL that Leadfeather has uploaded the first 3rd party Myth III map. Here's the announcement:

Leadfeather of the Vista Cartel and the Madrigal Chamber of Commerce is happy to release the first 3rd Party map for Myth III. A Place to Stand is a seven start ffa map. It features 2 daytime meshes A Place to Stand and A Place to Burn and two nighttime meshes A Place to Fear and a Place to H00r .

Leadfeather is one of the most talented map-makers in the Myth community and is even a part of the Vista Cartel team.

The map files are available at RampancyHL; more specifically at: Files> Games> Myth> Myth III> APTS_v1_1.zip and is 5.3 MB. Alternatively, you can download the map from Myth Village.