the 3 x 16 GB are probably different from the existing 16 single stick oem from Dell.  when you add 1 only, the bios was able to accommodate that by adjusting two different 16 gb.  when you add the rest two, now you have three identical non-Dell ram plus single Dell ram.  it was too much for bios.  nothing in bios setting can change that.  It is a relatively fastidious or non forgiving bios.  better get 4x16 that are identical and validated by users than a mix of oem and non-oem ram 1x16 + 3x16.
i just bought 3 16GB memory stick and its working when i add 1 into my R12 . but when i put other 2 the computer wont power on and the power key just blinking 4times each. do i have to enter bios and make any changes?
Aurora R12, three 16GB ram not working