Hardware:
Motherboard: MSI BIG BANG Trinergy. According to specs supports up to 16Gb of RAM. CPU: Core i7 870 GPU: NVIDIA 460 GTX
and three 100% idential memory sticks 2Gb (1333+) each.
However Windows only sees 4Gb.
I have run CPU-Z tool and it shows all three sticks.
Every stick works individually.
Intel Core i7 8xx supports only Dual Channel (two sticks). All three sticks would work with Core i7 9xx series.
Updating BIOS won't help, since motherboards support either Dual or Triple Channel. The best solution would be to buy an additional identical stick.
It may be that your motherboard is stuck in Dual Channel mode even though you have three stick installed (if you have an odd amount of ram installed and the motherboard supports it it should go back to single channel mode).
Have a look in your BIOS and see if you can force it into single channel mode or get a matching RAM module to pair with your third memory stick.
Dual-channel mode has to do with how the channels are used, it has nothing to do with the amount of memory that's visible. Yes, you're in single-channel mode (because the channels aren't balanced), but that's not why the system can't use your memory.
The system can't use your memory, most likely, because memory remapping isn't enabled in your BIOS.
