I have a brand new 15" MBP 2017, new Plugable TBT3-UDV dock, and two 23" ASUS 1080p monitors with HDMI support.
Monitor #1 is connected via DisplayPort-to-HDMI adapter.
Monitor #2 is connected via Plugable USB-C to HDMI cable.
My problem is that only one monitor works at a time. The first monitor I connect (either #1 or #2) works just fine while the second monitor acts like it is not receiving a signal. In other words, if I connect Monitor #1 first then Monitor #2 does not work. If I connect Monitor #2 first then Monitor #1 does not work.
Currently I am working around the problem by connecting Monitor #2 directly to my MacBook. However this defeats the purpose of the dual-monitor dock.