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You have Phosphorous (P) lock out , you can tell from the yellow color and the fact you've been using the wrong type of nutes. You should be using 8-4-4 instead. Hope this helps ! 🚀
Feeding every 3 weeks looks like it is not quite enough, feeding 4-8-4 should be safe enough every 5-8 days, especially now it is in flower.
Hope this helps,.........
Organoman.