Mine is working too ... got the new NUC in the store and tested it over there with no memory installed, but still no three LED flashes. It seemed strange to me that I would have received two defective NUCs in a row, so I took it home anyway. Installed the new memory (Kingston 4GB DDR3L 1600 MHz) and it booted properly, with immediate access to the BIOS. It seems strange that the manual mentions the three LED flashes when it obviously doesn't behave like this. Probably it was a RAM issue after all and my first NUC was also OK.
Got OpenELEC running on it in no time, everything works fine. Good to know the "insert stick after power-on" trick for if I ever get in trouble with the BIOS.