I've long wanted a portrait monitor for my Mac, frankly ever since I saw the original Radius Pivot display way back in 1990. I decided it was high time I had one. But I already have two monitors, and I was unsure if my system would support a third.
There is a lot of confusion information on the 'net about using three displays with an iMac. Part of the confusion is caused by different iMac models, different graphics cards, different monitors, and the usual amount of Internet know-nothings pontificating with apparent authority. I'm a professional in this field, and after hours of research, I was still only 80% confident in my conclusion that I could add a third monitor.
I settled on the ridiculously named "HP Z27n - Narrow Bezel 27" IPS LED Monitor - Quad HD". (K7C09A8#ABA) On the HP website, it currently sells for about $480. On Amazon, HP sells it direct for $382. Go figure why their website is higher.
Why did I choose this monitor? It's the size I wanted, I trust HP products, and most importantly -- it's one of the few models that pivots to portrait orientation. (Pro tip: Search for "swivel" to find displays that support this feature, that's the word most manufacturers seems to use.)
The result was successful The HP display works perfectly as the third monitor on my iMac. But as I mentioned, my iMac might not be the same as your iMac, so here are the details about my model:
I have an Apple LED 27" Cinema Display connected to one of the iMac's Thunderbolt ports. The HP display is connected to the other Thunderbolt port. Both monitor are running at full resolution/refresh. (2560 x 1440 60 Hz for the HP.)
This combination works beautifully. If you have a different iMac, different video card, different displays, or you want to daisy chain the displays -- you're on your own.