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I have a MBP 14,3 and wanted to connect it to some external displays via DP. I have the apple TB2 to TB3 adapter (the one that goes to USBC connector) and I noticed that while it will work with the Apple Thunderbolt Display, connecting a LCD panel to this using a DP to mini-DP cable won’t work. The monitor never detects the Mac.
I realized I had an old TB2 dock (El Gato) and figured that maybe it would do something amazing- and it did. Using that dock, with the TB3/2 adapter from Apple the Mac picked up the external display right away and worked fine. The one downside to this TB2 dock is that while it also has a HDMI output on it, it will not drive monitors on both the DP out and HDMI out. Only one at a time works.