As you may have found, documentation is here:
http://www.planetside.co.uk/wiki/Some parts are still a work in progress but we have a lot of great content there, and we're improving it all the time.
In addition, if you buy any version of Terragen 3, you get a full set of introductory video tutorials created by Geekatplay Studios. You can see the first few videos in the set here:
http://www.geekatplay.com/dt3.phpUpgrade pricing for all versions of Terragen can be found here:
http://planetside.co.uk/buy/tg3-price-listAs you can see, if you owned a license of Terragen Classic (0.9x), you can even get a discount on Terragen 3, and there is also an upgrade path to all higher versions.
Regarding the changes in user interface and workflow, it is certainly very different, but if you focus on the node list on the left side, you should find it fairly familiar from your Terragen Classic days, it works similarly. Think of the "Layout" buttons on the top (Objects, Terrain, Shaders, etc.) as corresponding to the left-side buttons in Classic, except instead of opening new windows, they take you to views of the node list and node network that focus on that particular aspect of the scene. But just like in TG Classic, if you go to Terrain, you'll see the things that affect your terrain (in the case of Terragen 3 you can have an unlimited number of terrains, instead of just 1, so it's more complicated, but can be used in just as simple a manner as Classic if you prefer). The main difference is that "Shaders" (like Surface Maps in Classic) are managed separately, in another Layout. Objects are of course a new addition. The other layouts follow similar approaches, with greater flexibility in the system meaning greater complexity (multiple lakes under Water for example), but ultimately a similar underlying concept. The new Node Network and its functions can be important with time, but you really can just focus on the node lists on the left and get quite a lot done.
Let us know if you have any further questions.
- Oshyan