actually crashcrash, your 3rd cell is way out of balance and is the likely cause of your problem.

cell 1 = 4.0V
cell 2 = 8V - 4V = 4.0V
cell 3 = 11.1V - 8V = 3.1V

your batteries need to be within 0.1V of each other or bad things happen, including poor performance. also, a lipo cell must never drop below 3.0V or fire may occur. your 3rd cell may easily be driven below 3.0V in flight with the power supplied by the 4V cells. you can use a commercially available balancer, or do it manually by putting a resistor (10ohm 5W wirewound) across the higher cells until they match the lower one, then recharge and recheck. repeat until they finish charging balanced within 0.1V. use the wiring code i wrote for you below to check individual cell voltage (and to manually balance them).

cell 1 -> black(-)/yellow(+)
cell 2 -> yellow(-)/blue(+)
cell 3 -> blue(-)/red(+)

btw, 99.9% of lipos around today are fully charged at 4.2V, which would make a fully charged balanced 3S pack have 12.6V. 11.1V is the nominal (or average) voltage. if you're curious, the other 0.1% of lipos are an obsolete variety that finished charging at 4.1V instead of 4.2V. they are no longer manufactured, and the standard is 4.2V.