This message was posted on behalf of Intel Corporation
Bear in mind that the processor on this Intel® NUC only supports DDR4 speeds of 2133 MHz, the memory that you are using is DDR4 2400M MHz. Have you tried using different RAM?
3 blinks without the RAM installed it's normal, those 3 blinks indicate that you are missing the RAM.
Check this link for System Memory for Intel® NUC Kit NUC6i7KYK, there you will find some 2400 MHz RAM that have been officially validated to work with this Intel® NUC.