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

baratorch,

Can you include this smackw32.dll file directly in the program?
Because where am I supposed to get it? The ones available online don't install.

smackw32.dll is part of the HoMM 3 game; this file is located in the game's installation folder. Your situation indicates that the HD mod is NOT installed in the game's folder (yes, strangely enough, HD requires the original game and must be installed in the same folder).

If, in your situation, HD is still installed correctly, try creating an HD executable in the launcher and launching the game from it.

Added 17 minutes later

We launch the game without the HD mod and load the save. We launch it with the HD mod and load the save again. When attempting to activate the game window without the HD mod, it crashes.
HD_CRASH_LOG.TXT

It's a pity that now you can't launch two copies from the same folder in single-player mode, as it was before, so you have to come up with ways to bypass this limitation.

Change the renderer in the launcher to the original 16-bit, and both the original and HD versions will work simultaneously.
I will remove this limitation. I added it because, in multiplayer, two instances cannot share the resources of one folder (crashes occur when passing a turn), and no one will fix this.
I will make it impossible to play online when multiple instances are launched from the same folder.