Panels in series have a voltage that is the sum of the individual voltages and a current equivalent to the smallest current. If several panels are in series and all but one have current of ten amps, and the remaining panel has a current of eight amps, the string current will be eight amps.
Panels in parallel have a voltage equal to the average of all the individual voltages and a total current equal to the sum of individual currents.
Placing all panels in parallel will allow measuring the individual currents but the charge controller requires three times the voltage for optimum performance, hence the series connection.
A electro-mechanical, or solid state, switch/relay could be placed in series with each panel and each panel electrically isolated in turn, for measurement. This would take a few milliseconds. The need for the data does not warrant such a step. Panels generally carry an industry wide warranty of twenty five years with derating. I may use one sensing resistor for the array in its entirety to check on the accuracy of the charge controller and as a general indicator of the array's status.
The equivalent circuit for a solar cell:
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