1) The Radeon HD 8760 is essentially a rebrand of the 7700 series for the OEM market.
2) You need at least one "active" display connection to enable the third monitor at the same time. Examples include:
- native DisplayPort display
- Active DisplayPort-DVI adapter
- Active DisplayPort-HDMI adapter
- DisplayPort-VGA adapter (all of these are active)
My guess is that the DP-HDMI adapter you're using is passive/