A leader is the most important role, although I've played successfully without one. Still, the game assumes a leader in terms of damage output math and so on.
Once you've got that down, you can make anything else work fairly easily. I'd say the second most important role is a striker, however; your damage output may be a little bit low given that a third of your party is not focused on damage. Choosing something else won't break anything, again, but it'll slow your combats down.
For the third role, go with whatever. Personally I'd choose a second striker, but that's pretty much personal preference for more damage output, and I wouldn't even try to influence my players by suggesting it.
When balancing three people, you'll want to look a bit further into the functions of the classes, as well. For example, if the first two are a warlord and a barbarian, you're going to be a bit more effective than if you went with a shaman and a sorcerer. Warlords and barbarians have good synergy, because the barbarian will take advantage of the warlord's powers, like the ability to grant basic melee attacks. A shaman tends to like to plant his spirit next to the monsters, whereas most sorcerers will be standing back -- so there's not as much synergy there.
Also, hybrids might be a very good choice here, unless your players are new to D&D 4e. Say, two striker/leader hybrids -- that way you'd have the healing you want, and when that healing isn't necessary they can focus on doing damage.