Consider this another vote for hirelings. Earlier additions of D&D had rules for hiring mercenaries and hirelings. Some of my fondest D&D memories are of adventures similar in feel to Jason and the Argonauts, or Krull, or Conan, or The Hobbit, or any of a number of other epic adventures where the heroes led a back of adventurers.
It's fun because hirelings die fairly often (they're all, at most, 1/2 the level of the PCs), which as any fan of Star Trek knows is a great way to heighten drama. Having the Big Bag guy ruthlessly slaughter a Red Shirt ensign is great way to get the players to invest themselves in the game--they'll want revenge.
Also, sometimes one of these hirelings will manage to survive. You can have them gain a level, maybe even retire. Now you've got a cool NPC contact for your PCs, complete with history. You can bet they'll be motivated to rescue the daughter of Johan, the guy that saved a PC's life back when they were exploring the Caves of Despair. Compare to the motivation to rescue some random NPC they've never met.
So: by all means, play with 3 PCs. Make sure you've got healing and arcane covered, and then just have the PCs hire out lower-level hirelings from the local adventurer's guild. Maybe even start the campaign with the PCs as hirelings of a higher-level NPC party.
Yeah, that could be cool. Have the higher level NPCs--famous adventurers, town heroes--lead the group to the Eeeevil temple, then say "you guys have done enough. We'll take it from here. You stay and watch the horses."
And then the NPCs never come back.
