Looks like nute burn to me. Or a defficiency cause by nute strength. Keep an eye on the new growth, real close. Are the tips tinged at all? At 3 weeks my ppm would not be over 400ppm using RO water. If the new growth looks good, It'll be fine. Go easy with the foliar feeding, just remember, these are weeds, they really don't need too many nutes to do well. The saying, "Less is more" holds true to these plants. Make sure your res temps are below 73 degrees. Make sure no algae is starting to form, you'll feel nasty slimey stuff when you clean your res, if it's present. I really like hygrozyme for hydroponics, it keeps the res in check and also helps keep the Ph from drifting up. Keep the ph between 5.5-6.3, I usually let it drift a few days before adding ph up/down.
I used to have problems when I first started, but after several grows, I got the hang of it. Don't follw anyones feed schedule, every plant is different. I've had some 5 week vegged white widows that did not like anything above 500ppm of Advanced Nutrients Sensi A+B during the whole grow. I've also had some that took 1200ppm quite fine. You'll eventually learn how to read the plants.
