3 weeks into flower is not good the normal yellowing is very late in flower not at the beginning of the cycle. The lack of chlorophyll will stunt the flower development at this cycle and the plant will not be able to produce the needed carbs and process stored energy it needs to survive.

What are you using as your base NPK nutrient?
What portions are you using?
What soil type are you using?
What is your Ph of the soil?