How to make an interesting 3rd level caster?

[Origomar]

I'm making a campaign where people pick from a pool of premade characters and the characters are mostly unaltered except for a few story related abilities/simplifications because they are newer players.

Everyones going to be 3rd level and they aren't going to level up at all in the campaign.

Problem is I can't think of many interesting lower level caster classes. I normally don't play casters so that's part of the reason, but also because most of the interesting stuff is higher level spells. The actual powerlevel of the casters isn't particularly important because since I'm making the characters I can fudge the numbers a bit to make them more appropriately strong. I just want characters that have somewhat interesting low level abilities without having to homebrew entire spells or abilities very often.

As of right now I have a wizard, an oracle and a summoner that I'm unsure of. I'd like the summoner to have a more powerful eidelon but not have access to summoning spells but I can't find anything like that(it's possible ive just missed it). Can anyone give me some suggestions?

[Barstro]

I'd like the summoner to have a more powerful eidelon but not have access to summoning spells but I can't find anything like that(it's possible ive just missed it). Can anyone give me some suggestions?

I've never seen that either; being able to summon for minutes per level as a standard action is something I doubt the authors would just trade away.

Since you want to, I'll make the following suggestions (even though I am against removing the utility of such summons)
1) Boost some Eidolon stats (boring)
2) Give the Eidolon some more evolution points (4?)
3) OR give the Summoner Lesser Evolution Surge at will.

I'm a fan of that last one.

[Psyren]

Most casters work fine at low levels. Summoning and rounds/level buffing tend to be weak because of duration, while blasting tends to be weak on damage, but that still leaves control spells like Grease, Sleep or Entangle, as well as minute-long buffs like Bless or Pro:Evil.

[Origomar]

Most casters work fine at low levels. Summoning and rounds/level buffing tend to be weak because of duration, while blasting tends to be weak on damage, but that still leaves control spells like Grease, Sleep or Entangle, as well as minute-long buffs like Bless or Pro:Evil.

It's not really about effectiveness as much as making them interesting to play. I don't want them to have to just spam grease or sleep because its the best spell they can provide.

[Kurald Galain]

It's not really about effectiveness as much as making them interesting to play. I don't want them to have to just spam grease or sleep because its the best spell they can provide.

Well then, (1) make sure they have second level spells, and (2) make sure the campaign rewards out-of-combat magic as well.

[Eldariel]

It's not really about effectiveness as much as making them interesting to play. I don't want them to have to just spam grease or sleep because its the best spell they can provide.

1) They have skills. Wizard can do a lot with all those Knowledges, Linguistics & co. for instance. They're actually walking encyclopedias: you can use them to tell the players a lot about the world, its history, its marvels and the creatures they're encountering.
2) Cantrips allow for a lot of cool stuff. Prestidigitation is a one hour "I can make the world into my image" for instance. It doesn't take a lot of creativity to make interesting Ghost Sounds or Mage Hands or such either.
3) Combat-wise, they have the most alternatives. Alchemical items give good touch attacks and damage, e.g. Alchemist's Fire. They can use crossbows (and elves bows) just as well as most warriors on these levels. They can make their meatshield grow (http://www.d20srd.org/srd/spells/enlargePerson.htm) or the enemy feel faint (http://www.d20srd.org/srd/spells/rayOfEnfeeblement.htm) (humanoids and carrying capacity). Look at this bulls-eye lantern, would you? (http://www.d20srd.org/srd/spells/pyrotechnics.htm) Perhaps someone wanted to scout or someone needs to hide? (http://www.d20srd.org/srd/spells/invisibility.htm) My, what delightful misery we shall inflict (http://www.d20srd.org/srd/spells/alterSelf.htm). Perhaps we should discuss the more in-depth details in private. (http://www.d20srd.org/srd/spells/charmPerson.htm)

In short, I don't really comprehend the question. Out of all the archetypes, it should be the least difficult to make a caster interesting. They can do a lot of different stuff within a classical archetype and their abilities offer a lot of space for creativity. Spells are the most varied options both in combat and out of combat. Just use a prepared caster, prepare a bunch of versatile spells and list some of their most common uses and let the players have their fun being creative with their little wishes