Grid Configuration

I've got a somewhat elaborate setup that I'm having trouble making work in the grid configuration. One of my machines (it happens to be set up as the primary, but I'm flexible on that) is a 3-monitor beast, the rest are single monitor. I have 6 monitors stacked 3 over 3, the 3-monitor machine is on the bottom, the top three monitors are run by three separate machines. My problem is that in the grid configuration there is no way to make my 3-monitor machine take up three grid spaces. Any ideas?

Thanks,
Matt