I was notified that there were small updates for HOMM I, II, and III today. I'm having trouble finding a changelog and was curious as to what the patches fixed/changed.

I think you mean the GL Wrapper.

No, the actual game in the library updated. GOG recently did multiple updates to many games, including old ones. It looks like housekeeping. According to Mortius1:
https://www.gog.com/forum/general/whats_with_the_absurd_number_of_updates_lately_i_smell_drm_rats/post10

I looked in the install directory of several of my updated games. Assuming I can trust file modification dates, the updates consisted of the following:

1. File Manifest, including Windows registry entries
2. Icons
3. Uninstaller
4. A script that appears to perform installation steps
5. The list of run targets used by Galaxy for "play" or "additional executables"

Witcher 1 additionally had a DLL update.