Hello everybody, tbh, this is first time I post 'article' here so I hope I post it on right forum. Sorry for my poor English too. I hope you'll understand.
To the point: Yesterday my friend and I bought Heroes. We wanted to play together, so I started a game, clicked on multiplayer - online and.. it crashed. I tried it again and it crashed again. Same happened to my friend. I found topics on google, people said something about using gameranger.
So we installed it, I made room, my friend joined it. I clicked on 'start'. Game started, but to me it said that cd is missing and friends game didn't start.
Anyone knows what to do? Thank you

I believe this is the solution you're looking for:

https://www.gog.com/forum/heroes_of_might_and_magic_series/homandm3_cd_required_problem

edit: I'm just giving a link since it involves regedits. Hate to mess one of those up.

Just for reference, I found it via a google search:

https://www.google.com/search?q=heroes+of+might+and+magic+3+missing+cd

Please let us know if that solves your issue.

I initially (just for a microsecond) misread this as "Yesterday my friend and I bought Herpes." XD

That's all, nothing to add (sorry, OP).