Question my ram

Jun 19, 2020
Im having a problem with my ram. I'm using 3 slots with 4gb module each one but windows shows only 4gb usable, my windows is 64bits and when I put only 2 modules I can use the 8gb or when I use the slots 1,2,3 I think it let me use 8gb instead of 4 but I want to be able to use the 12gb.
Now im using the slots 2,3,4 according the bios but according to the motherboard I'm using the slots 1,2,3.
Tried with the msconfig to start with "max memory size" but nothing.
Motherboard gigabyte 970a-ud3p
Ram 3 modules Hyperx 4gb DDR3 1866
I know my mother caps the mhz of the ram to 1666 if I use 3 slots but idk why Windows dont let me use it.