Optiplex 9101 All-in-one with three monitors?

I've got a new Dell Optiplex 9010 AIO. I have the built in monitor good to go, a 2nd monitor via a VGA connection flipped to landscape mode and also working fine.

I've got a HDMI-DVI converter plugged into the HDMI out slot, and plugged into an external IBM monitor. 

My computer can see it. I can extend the desktop between monitors 1 and 3 (as seen in the image) or 1 and 2 (the IBM monitor) but not all three at once.

Dell explicitly states that this computer with the onboard graphics card can support 3 displays (internal+2 external). I've rebooted, reinstalled the Intel video drivers and taken a peek into the BIOS, which didn't reveal anything.

Thoughts? Ideas? I can see the 3rd monitor through win7's control panel, and through the intel display management software. Just can't use all 3 at once.