Matewis: HoMM3 It's a phenomenal game, but it left me feeling exhausted after just 2 campaigns. The Shadow of Death campaign was listed at the top, and so I just naturally assumed it was the first one, and unwittingly jumped into the deep end. I managed to beat to 'A New Beginning' with Gem and 'Hack and Slash' with Crag Hack. Retrieving the cowel was hands down the most difficult level.
From a storyline point of view, The Shadow of Death IS the first campaign. It acts as a prequel to The Restoration of Erathia (and also is even set before Might & Magic VI: The Mandate of Heaven). In this campaign you learn for example why Gelu is the next captain of the forest guard (which he then is in HoMM3: The Armageddon's Blade (which is set after Might & Magic VII: For Honor and Blood and only minimally concerns the main story of HoMM3) and on a sidenote: The Story of The Armageddon's Blade is continued in "The Sword of Frost" an episode from the Heroes Chronicles which sets up the main story of Heroes of Might & Magic IV. Though HoMMIV has stand alone campaigns that don't even follow up one another and the game is just disappointing.
So it's logical to start with The Shadow of Death.
It's also the easiest of the 3 HoMM 3 campaigns since it lets you choose the difficulty you want to play on (the little chess figure) while both, The Restoration of Erathia and Armageddon's Blade have set difficulties.
HoMM 3 eats a lot of time. When i played it with a friend, we sometimes played a single mp map for 6-10 hours over the span of 2-3 daysm depending on how large it was. ONE MAP!
It's best to casually play the game and take your time. Don't rush through it like a mad man and also don't binge play it like the world is ending tomorrow. It can get exhausting if you play it that extreme.