Previously I was using 2 27" LG HDR 4K Display connected by USB 3, a 27" LG Ultra HD Display via HDMI and a 27" ACER via an Apple VGA adapter. After upgrading I can only reliably use two monitors. Sometimes all three monitors show up in the control panel but the display is black. Two lengthy discussions with support have not resolved the issue.
Mac mini 2018 or later
The 2018 Mac Mini Supports 2 display on the ThunderBolt 3/USB-C and 1 display on the HDMI.
see > https://support.apple.com/kb/SP782?locale=en_US
Unfortunately you will have to keep trying different configurations. Personally I would take the analog VGA adapter and/or monitor out of the mix and try using all digital adapters, cables, adapters and displays.
1, Try staggering the USB-C connected displays in opposing USB-C ports. (1 & 3) or (2 & 4) and see it that helps.
2, Try using a work around by setting up a "Put Displays to Sleep" Hot Corner.
Then if one monitor fails to wake, put all of the displays back to sleep and re-wake them.
It would help to know if it is always the same 2 that work or if it is random.
Thanks. The LG HDR 4K Display connecting by TB3 always works. When I boot up with the 27" ACER via the VGA adapter to TB3 and the LG via HDMI - only the LG HDR and ACER display. All three show up in the display control panel but the LG via HDMI reports "no signal" and goes to sleep. I tried a DP - TB3 cable on the LG (instead of the HDMI) and it works - but then the ACER doesn't. I think the mini only supports two monitors on TB3. My mini is a 3 GHz 6-Core Intel Core i5 with the Intel UHD Graphics 630 1536 MB. Running 10.15.1
Well - it's back to only two monitors working. After several days of all three working - the HDMI will not wake up. I replaced the VGA monitor with a new TB3 LG - staggered the ports and tried the sleep corner - the HDMI still is seen by the sytem - it shows up in the display control panel - but will not wake up.
Thanks - that seems to have worked (at least on the first reboot.) I haven't had a problem on waking from sleep but I did set up sleep corners just in case!
Great suggestion - just trying to save a few $$ by using my olde - but still working 27" VGA
You're welcome!
I believe that the 4 ports are powered by 2 channels.
1 for ports 1 & 2 and a second for ports 3 & 4
So, after upgrading to Catalina, I have lost the ability to run three monitors using the above guide lines
