Can it work without a tank? We've got a PFRPG'd Warlock, a ranger/rogue/shadowdancer and a wind Oracle, intended for stealthy capers but we have problems with combat. Maybe rebuild the Oracle?
Just give them opportunities to think their way through encounters, and plenty of escapes should they get in trouble. This can actually be more fun than bashing their way through encounters.
I also have a 3-PC party, but my problem is the opposite - way too much combat prowess. Barbarian, Monk, and an Oracle focused on healing. But because of that party, I have no heavy armor, no rogue-type characters, nor any arcane magic. I had to change the way I planned/ran my game. I definitely give them a good amount of combat, but I had to do away with excessive traps, and most arcane dealings besides magic items. I ended up setting my campaign more in the wilderness, just because it gave the characters more free spcace to roam and use their mobility.
You're going to have to change the way you plan your game, or at least plan for your characters to be able to use their abilities better -- Stealth, suprise, etc. Combat can still be a lot of fun, you just have to think of ways to make it so that the PCs aren't always trying to go toe-to-toe with foes. Or make it so that if they are, the foes they fight are on the same playing field (as in not using a lot of melee). It might help to ask the players what kind of game they want to play in. I know it goes against what you might expect your campaign to be, but it seems like they all have lightweight, stealthy chracters. And lots of combat might not be what they are after. And if it is, some sort of "out" or way to escape quickly would definitely help with players dying.
I also would never ask the Oracle to rebuild. If that's the character they want to play, then you're just going to have to find a way to make it work. Pidgeon-holing a person into playing something they didn't want to in the first place always leads to disaster.
As a side note, I've found that 3 is the PERFECT number for getting a lot accomplished in a game. There's a lot more focus on the characters, and their goals and agendas. And it's enough that they're not going to die every time they turn around a corner. Plus, in the pure combat aspect, it goes a lot smoother and everyone stays engaged because they get more "turns". It's one of the few times that they get to spend more time playing instead of waiting.
Most classes can "tank", it just depends on how you go about it. One of the most efficient "tanks" I've seen is a crane-style ninja using shadow clones, the only flaw is that he can't heal himself, but it's not like he has to all that often. Even sorcerers can tank if they use summon monster.
Thanks for all the advice. We'll continue as we are, sneaky and ROLE playing.
I find 3 players to be a good amount - combat moves faster that way. The GM just needs to adjust/create encounters that are appropriate. I never worry too much about covering roles in PF/3.5 or 4e. It's all about balancing the encounter to your group.
It is definitely possible to make it through a AP with three people, especially if you level them up slightly faster than normal. I can only think of one problem (you-know-who in ROTRL), but that can easily be fixed.
I'm the GM for this group. I'm definitely working on making encounters more tricksy and less smashy.
I think once we start doing more dungeoneering, they will work better in the confined area. They seem to get challenged when they accidentally combine encounters, or in wide spaced areas they can't rely on stealth so much.
Also, we realized the warlock, while usually filling the role of aerial artillery, can also be a very effective tank with her eldritch glaive (which gets iterative attacks and reach induced AoOs), DR 4/cold iron, and fiendish resilience (fast healing 2). She also has an eldritch barrier ability, which creates a 10 foot by 10 foot wall, which should be useful for tight spaces, battlefield control, etc.