You absolutely MUST pull the chicks from the nest box at different times, never, ever pull chicks younger than 2 weeks old, as they get much needed crop milk full of antibodies from their mother in the first 2 weeks. SO IF YOU'RE PULLING THEM AT 3 WEEKS OLD, THEN YOU NEED TO MAKE A CHART FOR EACH CHICK'S AGE, AND PULL EACH CHICK WHEN THEY ARE 3 WEEKS OLD AND NOT ANY SOONER. If you're doing it properly you should end up with chicks of all different ages that you are hand feeding, and that are all weaning at different times as well.
Do you have a brooder? All babies need artificial heat until they develop all of their down, or at least until they are 4-5 weeks old. You also need to mix up their formula differently based on their ages, you can't just feed all the chicks of different ages the same consistency of formula, as the younger the baby, the thinner the formula needs to be. Each baby needs to be treated appropriately according to their age, not all the same at once, that will surely end in dead babies.
So, pull each chick and put them in the brooder when each of them hits 3 weeks old, leaving the younger babies in the nest box with the parents until they are 3 weeks. And YOU WILL ALSO MOVE THE BABIES INTO THEIR WEANING CAGES AT DIFFERENT TIMES TOO, AS THEY WILL NOT ALL BE ABLE TO MOVE OUT OF THE BROODER AT THE SAME TIME OBVIOUSLY, NOR WILL THEY BE STARTING TO WEAN AT THE SAME TIME...everything is individual for each individual bird...
As mentioned, you don't have experience and you're best to not do this, just let the parents feed them, let them live in the next box until they are weaned, but just take them out and handle each of them every single day for as much time as you can, and this will hand-tame them while ensuring that they are appropriately fed.