Does anyone know if the 2017 5K 27' iMac can support 3 external monitors?

Mine is with Radeon Pro 580 8GB graphics, and can't make it work with 3 external monitors.


  • Retina 5K, 27-inch, 2017
  • 3.8 GHz Quad-Core Intel Core i5
  • 64 GB 2400 MHz DDR4
  • Radeon Pro 580 8 GB

My setup:

  • 2 x 1080p 24' HP monitors connected with hdmi cables to a thunderbolt 3 hub which is connected to 1 of the 2 thunderbolt port in my iMac
  • 1x LG ultrawide 29 connected with a USB C to HDMI cable to the other thunderbolt 3 port

The hub, cables and monitors are all working fine. However the 3 external monitors can't be used simultaneously, only 2 can be supported each time. Tried to disconnect and reconnect all the monitors with different orders, the last one (LG or HP) will always show "No Signal".

Is it because the iMac 2017 can only support up to 2 external monitors due to hardware / graphic limitation? Is there any configuration I can do to make it work?


  • What Thunderbolt hub are you using? I think that some of these Hubs can't support two video outputs at once, are you sure that's not the case here?
    – X_841
    Feb 24, 2021 at 12:22
  • @X_841, yes the Thunderbolt 3 hub works fine with 2 external monitors
    – Miao
    Feb 24, 2021 at 12:39
  • @Tetsujin, Apple officially states that the 2017 27' iMac can only support up to 2 external monitors, but my research indicates that the Radeon Pro 580 graphic card is capable of 4 displays and each thunderbolt 3 port supports 2 display drive (indeed I can run 2 external monitors with the hub which uses only 1 of the thunderbolt 3 ports of the iMac). So probably Apple is limiting it via software, I'd like to know if there's a way to unlock such limitation
    – Miao
    Feb 24, 2021 at 13:05
  • @answersun Both Thunderbolt ports are one controller. As you have discovered each port can support 2 displays, but not both ports. It is unlikely Apple limit it 2 displays purely in software.
    – Gilby
    Feb 25, 2021 at 6:30
  • @answersun This describes using USB for an extra monitor: apple.stackexchange.com/questions/293070/…
    – Gilby
    Feb 25, 2021 at 6:31


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