My understanding is that there are differences between the method used for dock detection. In the OG Droid, dock detection was through the use of magnets, in the Droid 3, it is based on electrical current supplied from a specific pin in the micro USB connector on the dock. This results in a proprietary usage/definition (read non-standard) of the pin connections on the 'dock'.
This type usage is true for both the Home Dock and the Car Dock. Neither dock can be used for data transfer between the Droid 3 and a PC.
If you have an OG Droid home/multimedia dock, you can 'mod' it so you can use it with the Droid 3. However, if you want/need the 'home dock app" to automagically (a favorite understandable non-word) start, you need a third-party app such as "Alarm Clock Plus". Search the Droid 3 forums for this 'mod'.
Bob