[Edi]

Most level 3 spells are useful, but some you really need to watch. There are also differences between spells of the same name that belong to different disciplines of magic.

To take an example, Black Death is available to Necromancy, Foul Magic, Nahualli Magic, Kuro Do, Troll Magic and Hedge Magic. It is a mass disease spell. In Troll magic it has an aoe of "all enemies". In all the others its aoe is "entire battlefield", which is obviously going to also hit the caster and all of his troops.

Undead are not affected by disease, so the only one who can use it reliably without shafting his own troops is a necromancer who has already transformed himself to a vampire or a lich. Given that disease has little direct effect in combat (well, Str -1, so less damage inflicted by those affected) but is otherwise much like Dom3 disease, the usefulness of that spell is much less than, say battlefield damage spells (especially if your own troops have immunity).

Then there is stuff like Meteor Showers which is a 1d20 blunt + 1d8 fire battlefield damage spell. It's great if you have tough troops of your own who can take the pounding and a caster who's either immortal or tough enough to survive and the enemy has more fragile troops. If you only have standard troops and a standard (i.e. fragile) mage, casting that spells is a surefire way to wipe yourself out pretty thoroughly. So use with care.

Whereas something like Hellfire (battlefield wide 1d10 fire damage) is great to spam if you have a fire immune army. Of course, if the other guy's army is also fire immune, that spells is decidedly less useful.

There are this kind of considerations in many cases. Also one of the things is that sometimes it is actually useful to memorize "useless" spells (such as poison resistance when facing units with no poison attacks) in order to curtail the choice of spells available for casting in a given battle. Then the random selection has a greater probability of hitting something you want, especially if there is one spell you would prefer the mage to cast over others.