2 8gb Memory Sticks in Dual Channel, or 3 in Single Channel?

[BlakeIng00]

My motherboard supports dual and quad channels, but when I originally built my PC I only bought one 8gb PC3-12800 1800mhz PNY XLR8 memory stick, as at the time that was the most I really needed to play the games I wanted to, now however memory demand for games is raising and I thought it was time to upgrade my RAM so I bought a 2 pack kit of 8gb PC-14900 1866mhz PNY XLR8 sticks. now my question is, would it be more beneficial to my computer to have all 24gb of ram, or just the 2 8gb PC3-14900 sticks in Dual Channel mode? As well if I use all three, would the PC3-14900 run at the speed of the PC3-12800, or would the 12800 try to run at the speed of the 14900's? I do have XMP on in my bios.
To be honest I'm not too sure of the difference between 12800 and 14900 besides the CAS9, CAS10 thing and I'm not too sure what those mean either.

 

[Tradesman1]

What mobo and CPU

 

[BlakeIng00]

Mobo: MSI 970A-G43
CPU: AMD FX-6350 currently at stock setting, normally overclocked to 4.2ghz though.
I had all 3 sticks installed, but I just removed the PC3-12800 because while it was in my PC would read all 24gb's of ram, but only 7.95 gb's was usable. All 16gb from the other two sticks are usable once the third was removed. If having 24gb would be more lucrative than 16gb in dual channel is there a way to get all three to work together?

 

[BlakeIng00]

I'm also running Windows 10

 

[Tradesman1]

Can try going into BIOS and adding + 0.06 to the DRAM voltage and set the CPU/NB voltage to 1.2, save, exit, shutdown....add the other stick and then give it a try

 

[BlakeIng00]

the CPU voltage is at 1.3645, am I supposed to lower it to 1.2 or was I supposed to raise it if it was less?

 

[Tradesman1]

CPU/NB it's a different voltage setting (feeds the MC (memory controller)

 

[BlakeIng00]

My bad, that's what I meant, I'd send you a screenshot but I'm not sure how?
so if its above 1.2 should I lower it?