I just figured this out - on the back of my graphics card, since it's a 2-slot card, one side has 3 ports - DVI, HDMI and DisplayPort, while the other slot was just another single DVI port.
In order to get the three screens to extend, I had to plug in all 3 of my monitors the one ONE SIDE of the card (I mistakenly was trying to run one monitor via DVI off of the second slot's DVI port).
So, the HDTV via HDMI, one Acer monitor via DVI and the other Acer monitor via the Accell DisplayPort-to-DVI active adapter.
That, and disconnecting all 3 screens and reconnecting them one screen at a time. Works great now - I can travel across all 3 with the mouse in whatever layout I like.