watchhowidoit is right, at 3 weeks she should not be taking nutrients from those fan leaves yet. A good flower nutrient with high P and K is necessary for flower development but some N will stave off those yellowing lower leaves until about weeks 7+trust me same thing is happening to me all the fan leaves are dropping i fed her but no result i guess the plant is sucking up its lower fan leaves for the lower brches of the buds to see light and to get extra juice into the top buds from those yellow leaves.