2KostyaN:

There is almost always a worthwhile use for the third hero's army:
the exception is when a Necro hero arrives third — it's harder to combine troops due to poor morale. But even then, you can sometimes come up with something. For example, if Galthran arrives third, you could consider making him the main hero.

Most of the points have already been covered here, so I'll add from myself:

the left army can often be useful when taking conservatories in the 1st or 2nd week, especially if you don't have a spellbook.

of course, fast units are used for speed boost (that's when you leave a hero to rest with fast units, and the next day he can move further); the terrain has nothing to do with it, really. There is no penalty in Dungeon, in swamp, or on lava. And even if there is a terrain penalty, it often makes sense to speed boost on roads.
The best option is to get Dragonflies for speed boost, as they are the fastest.

sometimes, when you need to clear a crypt or a dwarf dwelling on day 111, that army can also be very useful.

you can also use this army when breaking through shooters. For example, if the left army has archers, then when breaking through, the shooters will likely target those archers, giving the main army time to approach.

in the swamp, the left army can be VERY useful when taking Medusa hives in the first week, which are crucial when playing this faction. Despite movement penalties.

and finally, if the left army has Gnolls, Goblins, or Gremlins, those single units can come in handy if you break through to a neighboring respawn that is their native terrain. For example, you enter a teleporter and end up in snow-covered land. You don't want to take the main army into the snow, but a scout with a Gargoyle from the left army can collect a lot of resources there.

Good luck in the game.