Mac Pro 3,1 Using Radeon 5770 & 3rd Monitor for Logic movie

Hello all,

Please forgive this question from a bygone age ... but for the first time in years I have a reason to ask a question:

I have a Mac Pro 3,1 and used it happily with OSX 6.8 and my Logic 9 workstation which had a standard graphics card to drive 2 monitors. However whilst scoring a new film shot at 23.976 fps, the editors aren't converting to 24 fps fro post on this occasion, so I can no longer fire MIDI TC to a MacBook where the movie always ran out to a larger screen using a Canopus box (MIDI doesn't exist at 23.976 fps) and so I decided to purchase a ATI Radeon HD 5770 graphics card to fire out the movie to a 3rd monitor ... an LED tv screen. (The 5770 has 2 x DVI and an HDMI output)

Having installed the graphics card ... I plugged in the standard DVI monitor leads to my computer monitors which work fine ... but taking the HDMI output from the HD 5770 to the 3rd screen, there is no sign of the 3rd monitor working ... nor does it turn up in in the System Prefs Arrangement page or the Logic Cinema prefs also have nothing show up and "external video output" project settings.

I tried turning the 2 computer monitors off and re-starting the Mac up to see if anything turned up via the 5770 HDMI out ... but nothing appears?

Would a clever person know how to make this work? I've searched many forums and cannot find many with this request .. the spec says the card runs 3 monitors and there is an HDMI output on it?.

many thanks

Richard