Yes guessed as much, 3x4GB and 1x2GB RAM sticks. What are exact model numbers for the RAM sticks? As said before, mixing and matching RAM taht did not come as a kit/pack can be a gamble. Even from same manufacturere and identical specs. They cause various issue from erros and crashes, BSODs and sometimes in some systems they just outright refuse to boot.
What brand and exact model is your motherboard? I would take the 2TB and one of the 4GB RAM sticks out and check the system with 2 RAM modules in the proper slots determined by the motherboard manual (usually 2dn and 4th slots counting from CPU socket).
Not sure 8GB would be enough for Windows and the games though. But if you use all three of 4GB ones the system would probably run in single-channel mode instead of dual-channel, given the age of the system. Not sure you're board (don't know what it is) and your CPU support felx mode. But you might need all 12GB running Windows 10 and those games.
I would test using 2 of the 4GBs (dual-channel) for a while and see if crashes happen with some stress testing and keeping an eye on temps as well.
I would also test the RAM sticks, one at a time with bootable MemTest86 to see if any of them is bad and throws errors.