According to your manual (Section 1.7.2 part 8). The M.2 socket 3 is set as disabled. Refer to section 2.6.7 Onboard Devices Configuration and set the item PCI Express Slot and M.2 Bandwidth to [M.2 Mode] to enable this function.
In which case you go to the Advanced Menu. Scroll down to Onboard Devices Configuration. Then change PCI Express Slot and M.2 Bandwidth from [PCIeX1 and 2 slot at X1 Mode] to [M.2 Mode]. Then I would assume to save and exit then go back into the BIOS after reboot to see if the SSD is detected.
Manual:
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/Z97-A/E9378_Z97-A_UG_V2_WEB.pdf
By the way that motherboard only supports x2 mode for M.2. So you may not get full speed out of the SSD.