Sensing Light Differences in Photo-Resistors
At a high level:


Each LDR is set up as part of a voltage divider from supply to ground, with a fixed resistor as the other half of the divider, output from the junction
High input impedance difference amplifier between LDR1 and LDR2 outputs, and between LDR2 and LDR3 outputs
Analog adder, or OR gate, fed by the above two diff-amp outputs
If this results in a non-zero output, then either LDR1 and LDR2 differ in reading, or LDR2 and LDR3 differ in reading, or both. 


The difference amplifiers would need to be attenuated, to avoid very slight differences which are always present, from triggering this system.

Part numbers and  specifications would be decided based on the actual thresholds and LDR parameters. 



Update: To support the edit on the question.

Use a Wheatstone Bridge:







simulate this circuit &ndash; Schematic created using CircuitLab

If any single arm of the Wheatstone Bridge varies in resistance from the others, there will be a perturbation in the voltage reading. 

The initial effort to balance the bridge will be substantial, though.