First time to post, sorry if this is not the most appropriate place.

Joined GOG just to get HOMM3.

I have my laptop on bootcamp, partitioned with a Windows 10, but I primarily use it only for MTGArena.

I have most of my details on Mac. Can I buy then download on my mac, then transfer it to my Windows partition via usb?

There is a source port for HoMM3 that is called VCMI Project, and it enables you to run it on Windows, Mac OS,
Linux and Android. Just download the required Mac file/s and follow the installation instructions for Mac from the site:

https://vcmi.eu/

Wow! Thank you!

EDIT 1: Downloaded VCMI, but OSX Catalina won't allow to run the app. Sigh.

Is it more straightforward to buy on GOG, download and install on Windows?

Yes. Using the website, you can just set the System to Linux, download the offline backup installer, transfer the file(s) to your Windows partition (for example, via USB or a shared partition), then install it there. This can be done regardless of the OS used to purchase the game.

So, yes this should work.

(If you don't want to spend money on something you're worried won't work, just get one of the free games here and follow this procedure; with paid games the only difference is having to supply payment when purchasing the game.)