CPU/18 is 1802/18 = 100MHZ, that’s your problem.
You are correct, 64's don’t like more the 2 matched sticks. More so in the older 64's. If you are running 3 sticks you may be disabling DDR support.
Check your ram timings and force them to MANUAL and set it to 400MHz, or Ram Divider of 1:1.
If that doesn’t help;
Try taking the 512 stick out and ensure the other two are in sockets designed to operate in DDR and check to make sure your Bio's are set to Dual Channel support.
See if that helps.