You have three macro nutrients, Nitrogen, Phosphorus, and Potassium. Phosphorus is used for root development and aids in blooming. Since you are three weeks into flowering your plant is likely beginning to flower. Those flowers are going to use all the phosphorus they need and if you run out than you'll get a deficiency. I'm not sure if that is phosphorus deficiency in flower however, that tends to yellow and then curl the leaves.
I looked over my resources again and it still looks like a Phosphorus deficiency to me.
Try a 0-5-0 guano in a tea.