Hello Srolin,
The gpu_mask value is the integer representation of a binary mask.
For example, if you have 2 GPUs and wish to have both of them enabled, then the binary mask will be 11 (each bit applies to a separate GPU), the integer representation of "11" is 3.
So for enabling all N out of N graphic cards you can use the following formula to calculate gpu_mask value:
gpu_mask = (2 ^ N) - 1
For three GPUs, it should be 7, so it's correct, if you are using it for GTX 1080 system. You can also check the log on the node to see, if all cards are initialized in the beginning of GPU-supported stage.