Yea accod's spot on, no sff machine like the eeebox or the zotac zbox support 3 screens out of the box.
The Usb adapters are a stopgap solution better used for document viewing and web browsing than video or gaming due to the compression and transfer accross usb 2.0 limitations in place.... But it will get you to 3 screens , or 4 or 5 depending on how many adapters you purchase.
But again, at 3 screens you are likely going to overstress such a small machine.