I have a iMac 2009 later with High Sierra OS, I am using my mini display port already to 2 other tvs. I need to connect a 3rd tv but to display a different display. Only other ports I have available is a Firewire 800 port and 4 USB ports, I understand that the display driver for the USB ports on this Mac and OS version there was a problem and will never work. What other option do I have to run another 3rd video output to show a different display.
You dont say which 2009 iMac you have, but even the Late 2009 27" seems to support only one external display, up to 2560 x 1600. (From things like Everymac.com and MacTracker)
How have you managed to get two external displays running from 1 miniDisplayPort?
I doubt very much whether these machines have sufficient graphics hardware to drive 3 displays - 256Mb or 512 Mb Radeon 4000 series.
There may be third-party USB solutions, but they are likely to be annoyingly slow. It's USB 2, for starters, which does not have sufficient bandwidth/speed.