Look at what's in your water first before you add more CalMag. This definitely looks like lockout to me, especially with how small they are. Salt build up shows more severe signs in seedlings/clones as opposed to larger plants because the root-mass is smaller and can't take the same punishment larger root-masses can. Salt build ups essentially choke your roots, so clones/seedlings will show symptoms before larger plants do.
They're 3 weeks old, which means 3 weeks of that water being poured into the coco. Coco absorbs the water (and everything in the water) like a sponge. So if you have too many minerals in the water you're using (calcium, iron, etc) then once the water is gone whatever calcium, iron, etc. that doesn't get used by the plants roots begins to build up in the form of salts. After enough time, they will build up and begin choking your roots out and even causing pH fluctuations. Trying the CalMag will likely just make things worse. Excess minerals in water is no different than excess nutrients in water. Whatever the plant doesn't use remains in the medium as salt build up.
Change your water, don't water until the pots feel light when lifted, and lay off the nutes for a week or two until new growth shows signs of improvement. Fortunately they're still in smaller pots so it should only take one or two waterings to produce results.