I think a party of a battle oracle, a summoner with a strong melee eidolon, and maybe a evangelist archery cleric might do pretty well. Most of your bases would be covered.
Basically anything goes. I'd stay away from evil aligned characters, a party that is (mostly) good probably works best. Any character class should work, pick things you're comfortable with. A paladin is an excellent choice, certainly for a three person party: decent melee-tanking / great saves / some added healing / good social skills.
It really depends on how you want to run the campaign. The urban aspect allows for more roleplaying, if you take the trouble to look for it. If you want to do that, warn your players to get the right skills (and classes). If you're just going with hack-'n-slash, your players should choose appropriately.
The one thing to look for in a three person party is balance. You definitely need a healer, someone who can survive melee and someone who can deal some damage.
When I started my campaign, it looked like I was going to have three players. Since I'm putting a lot of emphasis on roleplaying and skills, I urged the players to play classes with a decent amount of (social) skills. We started off with:
- a bard;
- an oracle;
- an urban ranger.
The bard and oracle are good at social skills, the oracle is the healer, with the bard as support, the oracle is also a bit of a tank (high AC thanks to the heavy armor feat my player chose), the ranger is the damage dealer (two-handed) and has non-social skills that serve the campaign well. He also has a pet dog (I allowed him to take the druid rather than the ranger animal progression), which is a great way to get some extra help in combat.
Then we got a fourth player, who doubted between a paladin or a rogue. Both choices would have been excellent. He went for the rogue, great for damage and extra skills.
We do not have an arcane caster, which - I feel - is not a necessity for this campaign, but would certainly work. With just three players, you will probably be struggling a bit during the first few levels with a wizard or sorcerer, but you'll be fine after that.
Our group for the first two books was Bard/Summoner/Urban Ranger. That worked well, albeit the Summoner class seems inherently overpowered - too easy to use long-duration summonings for all sorts of activities, eg earth elementals to scout out the rooms of dungeons - the Summoner speaks Terran. The Eidolon can climb, fly, fight really well, etc. Bard and Ranger both seem effective but not overpowered.
If the party feels like it really needs help, consider one of them taking the Leadership Feat to get a Cohort to fill in where they need it.
Also, I'd advise having a Cleric (or an Oracle) in the group as this AP has several cases where status effects and none-hp damage come into play (Seven Days to the Grave and Scarwall especially). While Oracles and Druids can fit the roll somewhat, Clerics are better at adapting to the problem as they can either switch out their spells, or actually have the spells needed on their list.