Need a hand with Seamless monitor setup
2 PCs - 3 monitors each - 6 monitor setup
Have spent much time trying to figure the settings on this myself but need an assist. My monitor setup looks like this:

Primary:

SECONDARY:

I cannot figure how to grid or adjust settings to accomplish mouse movement from 3a to 1b by moving down through the bottom of 3a. I can only figure out how to get to 1b by moving off the right side of the screen on 3a. In fact I seem to be able to get every other possible movement combination except the one I need.
Stardock says no support for this type of question so I'm counting on this forum!
Thanks in advance!
Mitch