My babies are 5 weeks old now, but when I got them at 3 weeks, Theo was very small, compared to the other two, now Theo is BIGGER then the other two and the other two are pretty small - I assume as they eat more and more they'll grow a bit bigger just like Theo did.
Are you offering them veggies, fruit, seed at all? I started my 3 on veggies when they were 4 weeks old, and they don't exactly like them, but they don't really have a choice since they really need it, they need the vitamins and all that other stuff.
How many times a day are you hand feeding them? you're making sure (just gonna ask) their crops are full enough.. I'm sure you are but just thought I'd ask..
I just looked it up and got some of these that could cause it (doesn't mean it is): Infection, malnutrition, inadequate food intake, chick too cold, chick too hot, congenital abnormality, incubation problem, parasites.
Have a look at that site, it's got all sorts of things about 'growing chicks' and info, make sure they're kept warm enough and have some seed and water in the cage at all times. Someone else will probably be able to help you out more, but hope that helps a bit.