It becomes much more useful if your sovereign or champion has the impulsive attribute, since that gives a double move at the start of combat.

3 turn is really just unusable. The idea of a spell that just kills everything in an area after 3 turns is horrible. Even the spells that make you wait 2 turns feel clunky and unfun (not to mention glitchy).

Do not justify a spell with the horrible impulsive trait that really needs to be nerfed/removed. It should be an assasin trait only.

The idea of Multi turn casting is very sound. Counter spells goes well with that. The problem is that the battlefields are tiny and units have huge movement. This means that there is no time for maneuver and spell battles. 

I agree that there should be a casting time for more powerful spells, but 3 turns is way to long as most combat barely lasts that long.

Not true.  All they would need to do is make all non-instant spells cast the next turn with an initiative modifier if necessary.  this would allow the opponent to cast a counter if needed.

You could have counterspell be more of a "set this caster to counter the next spell attempted by the target unit"

I have said it before and I will say it again, spells should have a cast time measured in absolute time units, modified by initiative, not in full turns.  Turns is just not granular enough and can be spoofed by impulsive's double turn and haste's current "insta-turn" effect.  When you go to cast a non-instant spell, it should place a marker in the timeline with and icon for the unit casting and another icon to indicate the spell being cast.  I think they are working on some means of being able to tell when a spell is being cast so counterspell can work.

I agree that currently anything that takes multiple turns to cast has to be augmented by haste or impusive in order to even finish before the battle ends, which greatly reduces their usefulness.  I think I have tried one such spell without impulsive in a major battle, just to see how it worked.  It did middling damage to about two units that were left by that point.  You would have been much better off casting Storm or even Flame Bolt with those kinds of results.

I would prefer reduced damage and shard bonus instead of multi-turn spell casting.

When someone says a 3-turn spell is useless because most combats don't last that long, I don't think "well we should make all spells cast faster," I think ... maybe combats should last longer, at least 3 turns? The problem isn't cast times, it's tiny maps + too much damage vs. hit points, such that your melee can charge up to an enemy and kill him before your caster can finish that spell.

We may never have tactical combat with the complexity of, say, AoW in this game and I'm okay with that - but even in a game with a heavy strategic emphasis, running up and killing enemies before a spell with a cast time can finish is a little ridiculous. It's just not possible to have any tactics in that kind of "tactical" combat.

Just to be clear, Fireball and Blizzard have a cast time of 3. That means it takes four turns total, not three.

Fireball instantly kills trained units. Even hitting one is worth the cost. You need to plan a well designed Fire Mage before it becomes useful. I can get 4 caster turns before the enemy reaches my units. 

Fireball isn't meant to be useful in every battle. Use it in bigger battles with a lot of 2-move-per-turn enemy units, and preferrably with units of your own that can surround your casters while they cast. Obviously it isn't meant to be useful against panthers.

I think that the 3-turn wait after casting is fine. It's a damn powerful spell. And there is, as already noted, more than one way to counter it:

Fast-moving units, especially fast, high initiative units

Fire resistance

Archers taking out the caster before cast

Counterspell

Spreading units out to avoid multi-unit kills

And there are ways to make it more effective:

High-defense units to protect caster

Casting haste on caster

Slowing enemy units with various spells

If there are enough units, creating choke-points on the battlefield so the enemy can't spread out

Giving the caster movement bonus so they can retreat father before casting

I personally agree with your suggestions.  However the developers (I think Derek in particular) have expressed as a design principle that they want quick battles, no more than three minutes (and this presumably includes late game 9 vs. 9 unit match-ups).

(As an anecdote my most epic battle thus far with FE was the second battle that Procipinee fought in a 0.77 game.  She found no champions or useful gear.  After dealing with a Rock Spider which was nesting in a leather vest (phew!) she was forced to take on a Butcherman.  Before the battle she cast Regeneration/Evade/Courage.  She caste Haste on the first turn and then went toe to toe with the Butcher using her default Queen's Staff.  Combat lasted about 20 rounds (she did max 1 damage during entire combat) and was only won by two lucky dodges in the last 3 rounds.  She finished with 2hp's.  I was on the edge of my seat and actually cheered for the victory.  I also breathed a huge sigh of relief when we liberated the sacrificial dagger from the venomous villian.)

How about making the combat map larger, put in terrain features that affect combat. This would not only slow down combat a little so that spells with multi-turns will make sense but it would provide a more meaningful and enjoyable TC expereance.