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

Question: How do I prevent errors from appearing when starting campaigns? The campaigns themselves work, but there are no videos at the beginning.

As a solution: insert the game disc and uncheck the "No CD/DVD" option.
Or: copy the files heroes3.snd and heroes3.vid (totaling over 390MB) from the game disc to the _HD3_Data\CD Data folder.