Hi,
I've a N3. Since Onboard SDK 4.0 only Matrice 300 and 2xx are mentioned in the documentation, but not A3, N3 and Matrice 100 anymore.
It's easy to get the the code and the examples of 3.9 on github. But sadly this is not the case for the documentation on the development dji site.

How can I access the old SDK and the documentation of the SDK 3.x, please?

By the way, was there any anouncement that A3, N3 and Matrice 100 will not supported anymore?

Ok, found it: "This series of documentation introduces the functions of OSDK V4.0.0, as well as the steps and methods of developing program using OSDK V4.0.0. If you are still using OSDK V3.9.x, please download the documentation of OSDK V3.9.x."

As I know, A3 N3 and Matrice 100 are not supported anymore. Please check the release note. There are only M300 and M200 in support product.