It doesnt really matter how old the plant is, it matters what stage of development it's in. Hard to believe these are 3 weeks old, but even so they need to be treated according to their growth stage. These are still sprouts. One still has the seed husk on it.
If they are 3 weeks old and looking like this, there is a problem not related at all to nutrients. If I was to guess at a few things that would cause this, I would say:
Stop watering it
Dont feed it anything
Keep your dome on it
Or keep your humidity up real high
Make sure they are fairly warm 26c ish
If the dome and tray are dry, spray them
Try not to micro manage them. The most common new growers mistakes are overwatering and over fertilizing. Killing them with kindness, or smothering them with love.
Just do your best to keep their environment as close to what they want as you can get it. It will do the rest.
They can actually get quite big before they actually need nutrients. I personally wait until my first true leaves are about an inch long, by then there is another node popping up and another set of leaves.
That's what I'd say anyways