In the online game that I run, I have a party of three PCs. We originally had five, but two of them have had schedule conflicts and we've moved on with three. These are new players (and I'm a new DM), and we've been running through the Keep on the Shadowfell. I've actually detailed our sessions on my blog (links below), so you can see in detail the changes I've made to the adventure and the experiences my party has had.
The party is made up of a druid, a rogue and a swordmage. This means that we have no leader, and really no healing at all (one of the two players who had schedule conflicts was our cleric). Knowing that this would be a problem, I gave the druid the cleric's Healing Word power twice per encounter (same as the Cleric would have had), though without the Healer's Lore bonus. So, twice per encounter, the druid can use a minor action to let herself or a nearby ally spend a surge to get their surge value of hit points plus 1d6. Yes, it's overpowered for a druid, but that's okay - it makes the game more fun for the group and gives her something to do with her minor actions aside from switching in and out of beast form (which she rarely does).
I've scaled down the number of enemies in the encounters to try to make what would have been, say, a third-level encounter for five players (750 XP) into a third-level encounter for three players (450 XP). So, I'm removing 40% of the XP worth of monsters and traps. I've noticed lately that, as the players have gotten used to their characters, they're mowing down even-level and level+1 encounters without any trouble (partly thanks to some creative thinking on their part, which I like to reward). This is probably because they're slightly higher-level than it would look on the character sheet thanks to the free Healing Word. So, I'm starting to add a little more XP to the encounters, just to make them a bit more interesting.
I also remembered somewhat belatedly that I would need to scale down the amount of treasure that I grant, since the published adventure spreads it among five PCs and I'm only spreading it among three. This didn't get too out of hand or anything, but as the party graduated to second level, I started paying more attention to it.
Overall, it's been a fun game so far. The lessons I would share are:
- Don't be afraid to house rule an extra power to fill whatever gap exists in your three-PC party - it's technically overpowered, but it makes the game more fun for everyone
- Use the XP totals as a guideline to start with, and adjust as needed to tailor the challenge level to your party
- Remember to adjust treasure downward if you're using a published adventure that assumes a bigger party
Links to blog posts about the three-PC delve into the Keep on the Shadowfell so far: