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So, let's start with the fact that... regardless of whether you are a mage or a warrior, whether you have a Swamp Castle or a Tower, if your hero hasn't learned the skills of Earth Magic and Air Magic during the game, your chances of winning are practically 0 (without the Portal and Teleportation city spells, you won't be able to do anything against a human opponent). Therefore, these two skills must be present in every main hero if you are counting on winning.
Of course, Wisdom, Resistance, and Tactics are also 2 necessary skills that will be useful for both a warrior and a mage.
Here is a brief list of the most necessary skills based on personal experience.
Interesting... Well, I agree with Earth and Air, and I'll also add Wisdom, but I can't agree with "Resistance and Tactics are also 2 necessary skills that will be useful for both a warrior and a mage." - I usually stick to the idea that you need to choose skills that will help you counter the worst tricks of your opponent, for example, the Sphere of Prohibition. :) How many magical skills will be useless if your opponent uses it?
Logistics is a very important skill (especially for Necropolis, Swamp, and Tower heroes).
Well, Logistics, although it increases the number of moves, only gives a bonus to them on the road, so I think that Logistics without Pathfinding is not very effective. Question: why only for "Necropolis, Swamp, and Tower"?
I can immediately list the most stupid and useless skills in Heroes 3: Ballista, Ballistics, Mysticism, Scholar, Scouting, First Aid, Eagle Eye, and even Fire Magic can be included here... besides Berserk and Armageddon, there is practically nothing useful there, and these spells are used in extremely rare cases.
In short, if the game gave you something from this list, it's very bad :( although not fatal :) and in general, don't get into the habit of having your main hero wander around witch huts without proper preparation.
THAT'S IT! :D
I will immediately say that there have never been stupid and useless skills in Heroes 3!
You took "Fire"... I'm ready to bet that you never use the "Wall of Fire" spell, am I right? - It has saved me many times... And you can list the advantages of "Fire" for a very long time... ;)
"Единственное, что можно сказать с уверенностью об удаче - она изменит..."