Yellowing of the leaves is natural during the flowering cycle as you switch to a fertilizer that is higher in P and lower in N, so the plant uses up the old N stored in the fan leaves causing them to yellow. I don't know if it should be at only 3 weeks though, that does seem a little early. This link has pics of tons of plants with tons of different problems and a solution to each, it should help diagnose any problem you may have Plant Abuse Chart