Your gettin good info from these people according to what they see. Perhaps I can word some of it a little differently. At 3 weeks of flower the plant still needs a fair amount N to keep it green. All the leaves are not getting full sun The mag in epsom salt aids chlorophyll production in leaves not getting enough light. Helps to keep them green. Check this for a better understanding;
http://en.wikipedia.org/wiki/Chlorosis
You have been feeding it bloom food. Bloom foods are short on N. Short of N with a spike in P will also turn lower leaves yellow. Feeding good organic N can only help. It will not burn or cause any damage. Needing more mag at flower is also a very common issue. The suggetions for more N and Epsom salt can only help.