Hello everybody,
I was just wondering if there is some kind of mod that would enable me to recruit all available troops in Heroes 3, the same way as it is done in Heroes 4.

I don't know of any such mod. But I am not aware of all of the Horn of the Abyss' features and also Wake of Gods may have it (though I don't personally recommend WoG, it looks like a "let's put in everything I consider cool" kind of mod).

I don't think there is. BUT you can recruit them from the castle building, instead of trying to remember which buildings you've already visited.

I would advise against recruiting all units anyway. Not all units are useful, even upgraded. You get a bigger a bang for your bucks by saving your gold to upgrade a second or third or fourth town and making mismatched armies from recruiting the most efficient units in each of your towns.

I think there was 'Buy All' button in Wake of Gods MOD.