Strange drop in voltage on Eneloop AAA-Batteries
They were 2.86 v. Then I waited 5 minutes, measured the voltage. Now they were 3.1 v.


You may be seeing the recovery effect as the energy in the batteries is depleted to a low level. When the load is removed, chemical or physical processes inside the battery (e.g. diffusion) allow it to recover partly.

I would use an analog input to monitor voltage and prevent restart looping.

I would not expect 3 x AAA cells to provide long term energy. At the least I would try to measure power consumption accurately for planning purposes.