I just purchased a TBT3-UDV, along with a Cable Matters 103046 USB>HDMI adapter and a Plugable DP-HDMI2 adapter. I’m looking to run 3 external monitors on my 2017 Macbook Pro. The monitors are (2) Dell ST2421L and (1) Samsung Syncmaster TV. I hooked up the dock to my mac, then plugged 1 Dell into the Displayport>HDMI adapter using a brand new Amazon Basics HDMI cable. I plugged another Dell into the Cable Matters USB>HDMI adapter with another brand new HDMI cable, then plugged everything into the dock. The Dell plugged into the DP adapter came up right away, but the Dell plugged into the Cable Matters USB adapter did not. I went to the DisplayLink site and grabbed the latest High Sierra drivers, but this did not help. I decided to try a USB-C>HDMI adapter I had lying around (Cable Matters 201014), but plugging this into the Thunderbolt port on the Plugable dock did not work. When I unplugged the Cable Matters 201014 from the dock and plugged it directly into the Mac, the second Dell immediately came up. No matter how I try it, the USB>HDMI adapter seems to be a complete no-go.
So basically, I can’t get my Mac to see more than 1 monitor at a time through the Plugable dock. To get 2 external monitors (while the Macbook is closed), I have to plug one into the dock and one into the Macbook, which defeats the purpose of having a dock. Can you please help me get 3 monitors working? Failing that, at least 2 is a must. Thanks.