4e adventures are super-easy to scale in general. Just remove monsters until the XP value of the encounter is about the same as 3 monsters of the encounter's level. Usually this involves taking two monsters out.
That said, the monster design in the early adventures isn't great (and it's particularly bad in Gate Pass), so I've been repopulating the encounters from scratch using post-MM3 monsters. Also, ditch the tomb encounter entirely as RangerWickett said. It's a killer, and unecessary.
The biggest problem with scaling the number of PCs is dealing with the changes to action economy (specifically actions per round). I've found a lot of the suggestions Stalker0 put forth to reduce grind also reduced some of the headaches with less PCs. Specifically, never use enemies above the level of the PCs. With less actions per round and usually less people, the PCs will take more damage per person and deal less versus higher level enemies. It's a one-two punch against the players. I've found early WotBS to be particularly bad about this, probably due to the necessity of having to do this in 3.5/PF.
A lot of running the game with three PCs has to do with party composition. Depending on what roles you are lacking (and what classes you have), you'll need to adjust in different ways, and a lot of that is a learning curve based on your players.
You can also re-skin Torrent as a companion character depending on what role you need. Don't use her as written - she's far too complex to run by PCs or the DM, IMO. Pare her down to her basic attack, one at-will, one encounter, and one utility (or healing word) and call it a day. Personally, I'd go with:
Leader: Watermark/Melora's Tide/Healing Word Controller: Tidal Force/Crashing Wave/Ocean Shield Defender: Watermark/Crashing Wave/Power Melee (alternatively only the damage/weaken part of Refreshing Wave)