We usually end up running 3-man adventuring groups (4 total, counting the GM) and generally supplement that with 0-2 NPC's from the AP itself. That character or characters kind of serves as the GM's mouthpiece and often helps move the story along by providing needed insights or the occasional nudge.
Obviously going after a standard AP with just three people is challenging, but I have to say not only is it rewarding, but you get a LOT of playing done over the course of a session. We meet every week, but alternate between two AP's because we actually have four players, but two of them can only meet every other week.
We're just beginning the fifth book of Skull and Shackles right now with the main PC's being:
female Kitsune 11th level Sorcerer (Fey bloodline)
Our captain and clandestine lover and co-conspirator of Tessa Fairwind.
Half-Elven 11th level Master Summoner
Our ship's pilot - having an eidolon who can fly, breathe underwater and comminucate telepathically makes for a valuable scout while elementals of all sorts even the odds against foes on land, under the sea and in the air.
Human 1st level Unbreakable Fighter/10th level Invulnerable Rager
Sandara Quinn was a regular member of the party and a romantic partner to the Barbarian above, but left the group at the end of the book four on a personal quest for the Goddess Besmara - we hope to have her return soon.
We also just started the Anniversary Edition of Rise of the Runelords - it was made clear that this was a very difficult campaign, so we decided to do something different and run a group of sibling Summoners:
Half-Elven 3rd level Summoner
The oldest of the three, a half-brother long estranged from the family and recently united. He has a warrior's mentality and will soon fight from the astride his eidolon mount.
female Half-Elven 3rd level Master Summoner
Half-Elven 3rd level Master Summoner
Twins, these two went off in search of their rumored half-brother after their village was destroyed by marauding giants. They are united by the common magic in their blood.
Its still early yet, but it looks like our GM-run NPC will either be Shalelu or Ameiko if and when we get one.
At the moment we're already looking forward to the next AP we tackle - we're thinking either Second Darkness or Shattered Star. We're leaning towards Shattered Star since it will supposedly kind of echo what's going on in RotRL... in either case we'll probably run these three characters:
female Ifrit Sorcerer (Primal Elemental Fire bloodline)
Human Lore Warden
Its very early though.
Does anyone else generally run smaller parties through AP's? I'm curious what some of those experiences have been, what challenges came with it. In many of our encounters the creature make-up has had to be adjusted, but for the Rise of the Runelords campaign we've actually had our encouter difficulties raised to accomodate the early power of a well-built Summoner - we expect things to normalize a bit as we level up.
We had people drop out and had such a good core group that we just kept going with gestalt PCs. I don't really like GMPCs.
Human Rogue/Time Thief
The burly fighter left so the druid/ranger generally turns into a lion and tries to grapple when the going gets tough.
The elf causes some serious bomb chaos and can be more flexible with spells.
The rogue is even more of a stealth monster and doesn't have to worry about an armoured ally clanking behind.
They make up for the lost action economy by choosing the right time to 'nova' (use up bombs, combine wildshape with focus, time-thief combining haste effect and debuff strikes) and being a cautious group.
When the mythic playtest starts we're going to strip out the gestalt for the gnome and human and try mythic as well. We'll keep the Elf gestalt to compare.