Drooping leaves (looks sad), as well as some yellowing of leaves; although with my first grow and over watering mistake, I had also been accompanied by under-feeding so it was completely yellow from lack of nutrients, was growing in promix peatmoss. I'll let someone answer your nutrient question, I'm as experienced as you are with them, (none). You can find those symptoms from simple google search.
I was babying and watering every other day or so instead of just letting her dry to light weight and then water till runoff.
Actually, even if you have enough nitrogen, over watering can lock it out just like if your pH was off.
And OP-- if they are 3 weeks old its probably safe to try a 25% concentration of any type of "grow" or "veg" formula, usually recognizable by the high N rating, like a 10-2-4 or something. Mines pretty weak, but its organic so incan feed pretty often.
I'd give it 1 feeding with water, then just water the time after. If it seems alright repeat 1 more time the next 2 waterings and then give it 4-5 days to make sure it isn't too soon.
Once you can tell whether or not your plants can handle it (should take a week or less just to be sure) then you can start feeding more often (feed, feed, water, repeat) and start increasing the concentration. Your bottle might have the dose strength for seedlings.
