Adding a third monitor to my iMac 2009
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.