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

Does 3.28 connect with 3.28 without HD+?

Of course, it connects.

However, if the server is set up with HD+, and a player joins without HD+, it won't work.

Also, it won't work if you have a regular version (you are the host), and the opponent joins with HD+. This is indicated in the picture.

Pay close attention to what you are launching in the launcher.