efficiency problems connecting multiple solar panels
http://en.wikipedia.org/wiki/Solar_cell_efficiency#Maximum_power_point

If you draw too much current from solar cells their overall efficiency falls. This is evidently affecting all three configurations to different extents. If you test with a set of resistors in the 1 - 20 ohm range you'll get different results. Given the power dissipation it may be easiest to do this with automotive lightbulbs of different wattage ratings (higher wattage rating for same rated voltage = smaller resistance). Ideally you'd want an adjustable 20W dummy load.