Problem with triple monitors on Mac Pro

[hvhoog]

Hi, This is my setup:

- Host: Mac Pro with OS X 10.6.4
- VirtualBox version 3.2.10
- 3 monitors connected to the Mac Pro

The idea is that this way I can display 3 guest O.S.'s fullscreen on the 3 monitors. And this works... well, kind of.... When I navigate away from one of the screens, the display gets large black rectangles on it. This is only happening with VirtualBox and not with other applications. Previously I had only 2 monitors hooked up with the Mac Pro, and this was not happening... Here is an example:

[hvhoog]

I switched to VMware Fusion which can display VM's fullscreen on three displays without problems.