Not sure why you put the sticks the way you did, I can`t see the color of the ram slots from that manual.

But you should have the 2 4GB sticks in the 1st 2 same colored slots, whether that is slot 1 and 2 or 1 and 3. Most likely it`s 1 and 2, then put the 8 GB stick in slot 3.

Always use the slots closest to the cpu unless the manual tells you different. But if it indeed does say to populate 2 and 4 first, then leave it how you have it.