What boon for 3 PCs to bring them up to the power of 4?

[Dalamar]

I'd suggest just making them one level higher. I ran two characters, a wizard and a fighter/barbarian, through the Eberron adventure path (Forgotten Forge, etc.) and started them off at level 2 for the first instead of 1st as was suggested by the adventure. 32 pointbuy, too, I believe, and they were able to handle most anything the adventures had. The major contributor to this is the fact that the Eberron adventures have many fights with a group of weaker opponents that the wizard's faster access to area spells helped to combat, significantly reducing the effect of having less actions and bodies available.
The group did go through lots of healing potions, and later the wizard invested cross-class ranks in Use Magic Device to be able to pick up a Wand of CLW.

 

[Mark CMG]

A well-trained guard dog that will listen to anyone in the group that you level up as the group advances might be a possibility.

 

[Darklone]

Agreeing with Dalamar: If the opposition consists on more low level opponents instead of a few high CR monsters, the players will have an easier time to survive.