Hello spacecat84
Everything looks good according to the specs. I would suggest testing the 3 new DIMMs by themselves in slots 1-3. Also, check the specifications on the memory to make sure it can down-clock to 800 MHz. With all channels populated the DIMMs will work at 800Mhz.
If the 3 DIMMs work by themselves then test with 3 of the other DIMMs. With 2 channels populated the memory will operate at 1066MHz. If the memory works in both of those configurations then it is having an issue down-clocking to 800Mhz.
Thanks