Their appears to be little brow spots/specs scatte...

Well firstly I would recommend dialing down your nutrient solution thats super strong for a plant thats only three weeks old , secondly that looks like overwatering damages how often and how much water are you giving that plant is a question you have to ask yourself . What leads me to believe it overwatering damages is that the end of your leaves are curled down and the browning is coming from various places not in the middle not at the edges but all over and its not splots its patches , those patches will eventually turn super brown and dry up or they will turn black and spread mould spores . Its not a PK deficiency because your plant hasnt even started to flower ..... Its not a Cal mag problem because there is no red/purple stems and it wont start like that anyways it would be yellow inbetween the vains of the leaf . I would advise you transplant that plant into dryer soil and only water her lightly , and if you cant transplant her then raise the pot off the ground a couple inches and get air circulating around the pot so it helps dry up the medium faster , you gotta remember when they are young less is more .... I hope this information is useful to you on your quest to growing cannabis - Happy Growing!

Make sure it it not bugs. it doesn't sound like it is as you said only one plant is showing it but still good to make sure. Then you should double check the runoff pH next time you water to be sure it is in a good range an rule that out. Also leaves can get wind burn does that one plant happen to be the closest on to the fan? If so move the fan away a little or make it so the fan is not directly hitting it. as far as a deficiency its hard to tell because the newer growth seems to look pretty good from the pic/video👍. So i would try those things first and if no success message me and I will try my best to help out👊 Happy Growing Good luck