HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Are there any plans to support 3.59 in the foreseeable future?

Firstly, I don't see any demand for 3.59. I really don't want to work on something that only 3.59 people will use.
Secondly, Sergey Rozhenko and I had a conversation about the compatibility of 3.59 and HD. I was ready to work on compatibility, provided that 3.59 would use patcher_x86.dll to install patches and hooks (by the way, I believe that fulfilling this condition is not difficult at all), but this was not done.
Thirdly, what is this 3.59, and why is it needed? It's an alpha (or not, I'm not aware of the current status) version of the game, not even a base for modding.
What is the development pace? Have you finally added the city? No?
Then Era is a much more interesting base, on which dozens of mods are actually being developed.