Meanwhile, back on the magic-user spell lists...
Woohoo! Your magic-user has reached level 5! Third level spells! You have truly arrived. Or so we've always been led to believe. Let's analyze the Awesomeness Coefficient of these spells and find out how much truth there is to that.
1. Clairvoyance: It hasn't got quite the range I'd like to see in a scrying spell, but being able to see through the eyes of another creature behind walls or closed doors could be a very useful ability. You see whatever the creature whose eyes you're using sees, presumably including any special visual perceptions like infravision or magic detection. Thus, you can get a very good idea not only of the number and strength of creatures in the area, but the layout and contents of the room.
Clever characters could use the spell to discover secrets within a monster's or NPC's area. Watch for long enough, or at just the right time, and you might catch the villain slipping through the secret door to his treasury, or hiding the key in the mouth of the dragon's head trophy on the wall.
It might be most fun and useful in social/mystery adventures in strongholds or settlements. Need to know with whom the baron is meeting in his chambers, or what the merchant guildmaster is up to when he locks himself inside the vault? Clairvoyance is the go-to magic to get the scoop. If the party has a mole in some secret meeting, using Clairvoyance to look through his eyes is a good way to keep tabs on the situation.
Naturally, the spell conveys only what the target creature sees, and does not include sound or other sensory information. It lasts a full 12 turns, with the caster being able to switch creatures each turn if desired. As noted above, the range of 60' is somewhat meager. That puts it well within the range of other spells, though, and there's nothing in the rules to suggest that Charm Person or Invisibility can't be cast through a wall or a door. Clairvoyance lets you see potential targets without being physically present. If you see the orc king through the eyes of his bodyguard, why not try to charm him before you even barge into the room?
2. Dispel Magic: It's so simple and straightforward, yet possibly the single most useful spell a magic-user can know. It destroys all spell effects within a 20' cube, with a chance that spells cast by a higher level character are unaffected. Just about every party faces spell-casting enemies, and Dispel Magic is the ultimate counter. (Of course, the enemy will happily use it against PCs, too.) Any advantage that the bad guy caster and his allies can gain from spell effects - illusions, haste, flight, defensive barriers, invisibility - can be wiped away in one fell swoop. So can effects that hinder the party, like sleep, charm, hold person, web, wizard lock, being blinded by light or darkness, and such. Alert players might even be able to catch an opponent at a point where losing the enchantment would be really inconvenient, like while levitating 30' above the ground or walking through fire under the influence of a Resist Fire spell.
The range of 120' is quite respectable. I can think of only two real weak points. It isn't much good against spells with an instantaneous duration, such as most direct damage-causing spells. Since it's an area effect spell, and can't be targeted to a single creature, care must be taken, lest the caster dispel beneficial effects from his own party members.
3. Fire Ball: This was always the spell that every magic-user in my campaigns of yore lusted after. Big damage - a d6 per level of the caster, and over a big area 40' across! It doesn't hurt that you can fling it up to 240' away, either (so you can damage an opponent at 260', taking the spell's maximum range as the center of the fiery blast.) At 5th level, the damage rolled averages 17.5 points, which is just short of enough to take out an average 4 HD creature, assuming saving throws are failed. That's pretty potent, and it only goes up from there. At level 10, a magic-user is dealing out an average of 35 points, and he can do it three times per day if he uses all three of his 3rd level spell slots for Fire Ball. The average 8 HD creature has about 36 hp.
My players tended to save big-gun spells like Fire Ball for the biggest enemies, and it certainly is good for softening up a really tough opponent before the fighters close to melee. It's perhaps even more useful for wiping out lesser enemies en masse, turning a protracted, resource-depleting combat into an instant rout, allowing the party to advance toward a bigger goal without sacrificing any of their precious hit points.
As powerful as Fire Ball is, its limitations should not be downplayed either. In a dungeon, a 40' blast area is bigger than many rooms. True, you can cast it so that up to half that area is "wasted," expended against a wall, but that's still engulfing a 20' radius semicircle in searing flames. Besides monsters, you might be inadvertently frying stuff that you might rather like to pick up after the fight, too. Outdoors, it's likely that enemies will be spread out a little more, so that 40' blast will only catch a few. (As a comparison, the three-point line on a basketball court is twenty-ish feet in radius, comparable to the area of a fireball. Put ten guys in there running and jumping and bumping into each other, and it starts looking pretty crowded. Disciplined troops in a large army might charge in tighter formation, but ragged skirmishing groups of bandits or humanoids probably don't.)
Using Fire Ball as a one-shot win against a formidable host in either setting should require either clever tactics or very good luck.
4. Fly: For pure freedom of movement, it's hard to beat this one. It allows flight at up to 120' per round - that's three times the unencumbered encounter speed of a character, or equal to full running speed, with no real exertion at all - the magic does all the work. (It has to, right? It's not as if the magic-user flaps his arms or otherwise expends physical effort to propel himself.) There is no stated weight limit for Fly to be effective, so that's open to interpretation. For the sake of simplicity I'd probably rule that up to the maximum weight allowance for a character, it functions as normal, and beyond that it's too much for the magic to lift. It lasts 1d6 turns plus the caster's level, so even at 5th level, you get at least an hour of flight, and it can be cast on another creature by touch if desired rather than on the caster himself.
Except in really enormous rooms with high ceilings, Fly is of very limited utility in the dungeon. Outdoors, the sky is literally the limit. I could list all sorts of advantages that a PC might gain from this, but frankly, if you can't see them, what are you even doing playing a game of imagination? I also can't see any reason that Fly wouldn't propel a character through water as well, though perhaps at a lesser speed due to resistance.
So far, I'd say these are all living up to the hype. Can this 3rd level spell list sustain that level of greatness for another eight spells? (Spoiler alert: Yes, it can.)