From the iOS header file for data packet type
* Packet contains information about signal quantization.
* Size of the values array for this packet is always equal to the size
* of EEG packet and has the same channel mapping.
* Each index in this packet corresponds to the same index in an EEG packet.
* Quantization occurs when there is a particularly noisy signal, which
* generally happens when there is not a good contact between the headband
* and the skin.
* Higher numbers are worse; 1 is no quantization, and 16 is maximum
* These values are used under the hood by the library and by %Muse Elements
* in reconstructing the EEG signal and contributing to an overall measure
* of noise; it is extremely unlikely that you will be interested in them.
* For measuring noise, it is recommended to instead use the more useful
* computed values like 'headband_on' or 'horseshoe'.
* Each quantization packet applies to the next 16 EEG packets.
So my best guess would be that size 3 means 0,1,2,3, one for each of the four sensors.