Three's a Crowd

[FrozenChrono]

I run in 3 person parties regularly. I actually prefer them. There's a few things you need to keep track of though.

Whichever role is missing take that into account.

-Fewer minions if there's not a lot of control,

-Give them lots of healing potions and rest if there's no leader,

-Steer clear of +level solo's and elites if there's no striker

-Lower the damage output a bit if there's no defender.


Some additional tips.

Solo's can still be used, just make sure they are equal to lower level than the party. A same level solo is going to be a big challenge for 3 guys.

It's harder to to big groups of enemies but don't be afraid to pull out a few -1 to -3 level baddies so you can have them fight 6-8 regular guys instead of 3. keep track of to hit when doing this because if you go too low it's easy to make the encounters trivial.

As mentioned, stun, weak, and daze effects become much more powerful the smaller the party.

 

[Dragonblade]

I currently DM for two players. I run published material as written with the following house rules to help them survive:

84 point buy for stats, 1 for 1 (not the WotC costs), starting from a base of 0, and no stat can be less than 8 or higher than 18 prior to racial modifiers. This is intended to allow for two or even three 18s so they can have a good score in every ability score their class uses.
+2 to saves
+1 square of base movement
Double HPs

They also get 1 Fate Point per Extended Rest (and never more than 1) which they can spend as a free action any time to instantly end any one condition currently affecting them, or to heal as if they had spent a healing surge.

I don't usually reduce the number of creatures, but I do reduce monster HPs from 25% to 50% depending on how tough I think the encounter will be for them.

These changes are designed to essentially let a party of 2 be as tough as a party of 4. So far it has worked out well.