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Your capacitors have slightly different capacities. This leads into slightly different voltages at each of the three capacitors. After discharging they have different residual charges. This residual charge is added to the fresh charge and the capacitors have different voltages. May be this exceeds the voltage limit at one of the capacitors.You should put a resistor e.g. 10 kOhm in parallel to each of the capacitors. This makes shure that all of them will be discharged totally. May be a total of 6.9V is big enough that there is no problem.If you use accumulators this is done by a load balancer. But in this case some resistors will do the job.
very very very great job!!! indeed!