I've been playing with a 3-PC party: Fighter, Ranger, Warlock. The fighter is multiclassed into Warlord and is really the only source of healing outside potions. They're up to level 4 now, and it hasn't been -too- bad, but here's some notes:
Be fairly generous with healing potions in treasure, such that they have a few to toss around the party at any given time. At low levels, the potions are really quite good and can easily make up for the lack of a true leader class.
Be a little careful about encounter design. Don't go way over the xp limit. Brutes can actually be OK since their attack bonus isn't usually too good, but beware of anything that deals ongoing or automatic damage. Some of the low-level swarms are actually quite nasty, a few bad rolls with those can quickly turn things ugly. Using multiple soldiers against the party will probably lead to boring grindfests; use a soldier monster to protect a soft squishy monster instead. Minions are very fun against small parties!
My guess is your Ranger will tend to run out of healing surges way before the two defenders, although Lay on Hands may mitigate this. You might advise the Ranger's player to invest in the Durable feat. Keep in mind that they may not be able to handle more than 4 encounters per day, and that if things go poorly the Ranger might well be out of surges after only two tough fights, so it might be good to break a dungeon into chunks where they can rest in between.
But overall, the game scales pretty well down to 3 players. The hardest part for me was finding enough suitable low-level opponents to keep things interesting, but you can always scale down higher-level monsters a few levels using the rules in the DMG.
The only special thing I've given the players was to bump any still-empty slots up to +1 weapons/armor/neck item at level 3 more or less automatically (I have an in-game explanation for this).