I have the HP Envy 17 A6U49AV, running Windows 7 Home Premium 64 Service Pack 1, Graphics 1: Intel HD Graphics 4000; Graphics 2: Radeon HD 7850M. Trying to extend the desktop across three ASUS VS228H displays.
Have plugged in three monitors, with notebook lid closed. Display 1: HDMI port, Display 2: Passive DisplayPort dongle to HDMI, and Display 3: Active DisplayPort to DVI. Only getting two monitors working at once - can get any two working but not three. Or, one monitor with the notebook display. Bios and Radeon drivers are up to date. The video that seems to explain all this at http://support.amd.com/en-us/search/faq/154 seems to indicate the powered adaptor is only needed for the third monitor.