A cap won't help. Your problem is that there is too much constant current drain. A cap will only help with bursts of current draw. If you are using 6 cells, your best bet would be to use a
LM7805 regulator for each fan since each fan draws ~1A.
Be warned, 2A (total) of fan current will affect runtime.
Or better yet, design the setup so a fan is not needed at all.