Koepel:
Using the Arduino Mega 2560 board at 3.3V didn't ring a bell before, but it is not possible.
The Arduino Uno uses a ATmega328P which will probably run at 3.3V, although not according to the datasheet.
The Arduino Mega 2560 uses the older ATmega2560, and that chip must have at least 4.5V to run reliable.
I thought the ATmega2560 operating voltage was 1.8V - 5.5V? Can't I just lower processor speed?
Koepel:
If you have already connected digital pins (with 5V HIGH level) to the AD0 of the MPU-6050 chips, you have to throw them away. All of them.
Why do I HAVE to throw them out?
Koepel:
Can you tell what your project is ? Why do you need 14 accelerometers/gyro sensors ?
The project is a motion tracker as input for a robotic hand.