eneough light? sometimes the ammount you got aint eneough or Vitamin B seriously you cant go wrong diluting some of that and giving it to them when they droop. Mine get yellowing leaves but not that young unless they are clones and ive snipped the ends to promote growth.

You say they are exactly 3 weeks old...but you got them three weeks ago...? did you get them as seed? or clones..?

Can the roots breathe? Is the soil loose? is it packed? Too much water? It doesnt hurt them to let them dry out. Too much water will kill them just as surely as too little.

Hope I can help friend would hate to see anyone's grow go to crap