HOMM 3 had challenging campaigns and a difficulty that was perfect, as well as a damn fine sense of balance. Impossible difficulty makes you stress the importance of EVERY SINGLE DAY. Build orders are more important, all visitable locations aren't stupidly hard or completely useless like HOMM5.
HOMM5 was a monstrocity to the series. Ubisoft took a good game and stabbed it several times, turning it into a generic system (Much like the Disciples series with Disciples 3, although the graphics look nifty). The combat system/spells in HOMM3 are what make the combat -- completely unique at every turn. You could replay a close fight 50 times and the chance of victory would be 50-50 every time, depending on when you use spells and who you use them on. There are also a large number of towns, all which have overall specialties and weaknesses (eg. Fortress have the best t3 units and have a LARGE defensive bonus, but suffer from ranged attacks, Stronghold have some of the best units + massive offensive bonuses, but only get lv3 spells). The only matchup that is imba is Necropolis vs Conflux, as Vampire Lords (best t4) can't suck life from elementals and Necro's also suffer from lack of magic defense.
I've been playing HOMM3 for 8 years now and I'm not bored of it yet. There's still a strong community at celestial heavens which swears by it more than any other in the series. But to be honest, the highlight of the Might and Magic franchise comes from Might and Magic 6. This is probably the best RPG i've ever played, with maybe a slight allowance for Baldurs Gate 2. The graphics are terrible by todays standards, but if you want a game that is rich, deep, challenging and has amazing loot with a well rounded storyline (which coincides with the events of HOMM3 campaigns) then it's definitely worth a try. Most websites will have it rated in the top 5 RPG's.
Unfortunately, it uses sprites which you don't seem to like all that much
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And I disagree with sating HOMM1-5 were the same. 3&4 are almost entirely different games in terms of both combat and map exploration. 5 is a dumbed down version of 2 and is incredibly slow and unbalanced.