to be totally honest man, i'm so lost with where you're at with your grow. you should just start in mg, if you're going to be using that. transplanting and changing soil this often, this young is only going to cause stress to your little seedlings. if they're growing, let them be until they get a bit bigger. these are plants, they take some time. don't rush things. it will take a few days to see any major changes for better/worse after you do something new. if that's what your plants are at at 3 weeks, there's something awry. what i would do (i had to do it) is just start all over. i had 5 seeds that had cracked, not one had broken the surface after about 6 days. even if they would have, probably would've been some weak plants. so i chucked em all, re-evaluated and started over. only use the mg soil, what kind is it specifically? if you can get your hands on some dolomite lime and some pearlite, add that in. sphagnum peat moss too if you'd like. soak the soil with RO/distilled/set-out tap water before you put the germ'd seeds in then only water when the cup is light. use the 18oz cups to start in, and then transplant when they've got maybe 3 or 4 nodes. if you can make a humidity dome thing over the cup, to keep it a bit more humid in there, that'd help. but remove it as soon as it breaks ground. i don't know man, i've been trying to follow along, but i'm kind of lost..and to be honest, i think that's going to be the best thing for ya