I think you might actually be better off with the cleric than, say, a fighter anyway. I'm running a three-char party with a fighter, ranger (mostly bow), and starlock. And no matter how hard the fighter tries, after two or three encounters at least one of the strikers will have taken some lucky shots from skirmishers/artillery and have to burn through half their healing surges, while most monsters can hardly touch the fighter (who has 12 surges anyway).
The fighter's multiclassing into warlord to ameliorate some of this, but the party's still only level 3. Often I think the party could use in-combat healing a lot more than the tanking the fighter provides, but this could be a "grass is greener" situation.
I think you should be OK, just make sure they have plenty of heals ready for the Wizard (healing potions are fantastic), and advise them to take out the artillery first. And perhaps be sparing with brutes. The other thing to watch out for is keeping an encounter's xp budget reasonable; with only three characters it's easy to accidentally add a monster/trap and wind up with a much harder encounter than intended.
But in the end, I've found the three-char party works surprisingly well. Good luck!