Is it a Cal-mag deficiency? From total newbie point of view.

At that stage, it's really difficult to determine. You're going to have to let it progress more for there to be clear symptoms. Some of your pix have blotchy yellow spots beginning to develop between the veins of the leaves, which could be early calcium deficiency, but there's no necrotic spots yet. Those plants are very early into their growth cycle and 3 weeks in it's not really super common to be calcium deficient unless your regiment is completely void of calcium. But if you have hard water you are getting calcium to your plants roots. Even soft water has some calcium and magnesium. Are you using RO or some other demineralized water? Dialing in Humidity and temperature is vitally important for your plants health because it dictates the nutrient uptake rate. Lower the VPD the slower the transpiration rate, which means the slower the plants uptake water and nutrients and expell it through the leaves into the atmosphere. The higher the VPD the faster. You can feed your plants conservatively but if your VPD is high they can uptake at a very fast rate. So fast that it can cause your leaves to burn. So VPD is really important to dial in and maintain. Low VPD during veg, like 0.85 to 0.95 is ideal. You'll achieve that with a temperature of around 78F and 65% humidity. VPD for flowering is roughly the same temperature, but with a lower humidity. About 1.60 to 1.70 VPD is ideal during flower. To achieve that you want to drop your humidity to between 40% and 50% with a 75F temp. I'm not familiar with your nutrient line you're using. But i looked at your diary info and everything looks good as far as temps, humidity, and light distance. They don't appear to be over watered. I would advise to leave it be for a few more days and let it progress a bit so that more signs and symptoms are present. Then post new pix with a new question. Right now it's really hard to tell whats going on. It could turn out to be nothing major. Flushing your soil when issues happen often helps, but i wouldn't advise it yet because at this point I think it's too early to say with any degree of certainty.