well Ballowers, when it comes to might & magic, im am probably this forums foremost expert, so I would be glad to answer your questions
yes, the HD steam version is missing the two expansions, along with all the features, units, spells, artifacts, heroes, and buildings they added.. they claim they lost the source code, but I think thats hogwash and they were just looking for a quick cash grab with little effort..
if you want to play Heroes 3, here is my suggestion.. first, buy it off GoG since GoG has all the expansions included, second download the fan-made HD asset pack as its sprite quality is much better than steams "HD" version.. also, after you have played its campaigns, try the fan-made expansion "Horn of the Abyss", it has a new castle type (the Cove, pirate themed, meant to represent the Ragnan Empire which is a major faction in the lore, but only ever really appeared in one game), units, spells, artifacts, building types, and a new type of Siege weaponry..
the Might & Magic games were originally created by a developer named Jon Van Caneghem and the studio he opened "New World Computing", it eventually got bought out by 3DO who ran it into the ground and gave them almost no funding and unreasonable deadlines before filing for bankruptcy putting Jon and his team out of work.. Ubisoft quickly bought the rights to the IP
part of the draw to the series was its lore and stories, lore deeper and more complex then even the Elder Scrolls has.. each games dialogue referenced previous games, foreshadowed the next, and just gave you a basic idea of the worlds current political state (similar to TES)..
but when Ubisoft took it over, they threw all that out the window saying "yeah, none of that exists, we are going to make our own fantasy setting and slap the Might and Magic name on it, that way it will sell based purely off brand recognition!".. however, their setting is generic as they come, and the writing is as if some freshmen high school student watched part 1 of the hobbit and went "I can totally write a fantasy story!".. they threw almost 2 decades of lore and world building out the window, got rid of the interesting mysterious concepts like the Kreegan, Ancients, and Guardians to replace them with some of the most generic stuff ever..
seriously, Ubisoft games have no connection to the originals at all, nothing, the only thing they have in common is the name
on the heroes side of the franchise specifically (Heroes being the TBSRPG half of the franchise), it resembles almost nothing like what it was when NWC had the IP..
Maps used to be large, with countless ways to go to search for your objective (for example, ones objective on a map may be "find and capture the Barbarians Capital", but you don't know where it is so you have to go out and search for it or try and find clues), but now they are as linear as they get ("you start at this end of a road, your objective is on that end of a road, there is no way off the road, go!")..
unit morale is near non-existant in the new ones (in the NWC if you had units that were of a different allignment (those being good and evil) in your army, say a few Angels and Devils, they would have a drop in morale, while units of the same would give you a mild buff in morale, there were also "neutral" alligned units who woould not affect morale at all), magic was nerfed entirely to the point its almost pointless, same with your heroes skills, the games now have almost no difficulty compared to the level of challenge you could get in NWC's games, there are far less artifacts to find, and the types of buildings on the map are far less than they were in NWC..
so yeah, Ubisoft's games are "Might & Magic" in name only, because they completely ignore what Might and Magic had been about since it first appeared with the first game "Secret of the Inner Sanctum"..
still, I am so happy that GoG has the REAL M&M games available, and I always tell people they should go check them out
DISCLOSURE: I apologize on the wall of text, i am just really passionate about what the franchise was, and not what ubisoft turned it into..