my question is regarding galvanic isolation between three photovoltaic modules
You can use a series diode per module to semi-isolate them.  

For heavy shading on one module that module will have lower voltage on the module side of the diode.

For light shading the overall panel voltage may still be high enough to allow the module to still deliver some power.  The voltage drop across the diode for the shaded module will usually be lower than for unshaded modules BUTas shading increases the opposite is true.

More accurate is to measure the voltage drop and so the current across a series resistance per module. This could be eg the feeder lead or a low resistance precision resistor. eg at say 10A a 10 milliohm sense resistor will drop 0.1V. 

Often not an issue but note that for very long strings that give very high voltage, shading a cell that is not diode protected will lead to high reverse voltage across the shaded cell, which can produce cell catastrophic breakdown in low quality low isolation cells. Fire can happen in such cases.