After reading through these and trying numerous attempts at various orders of plugging and un-plugging my monitors, I was able to resolve this by simply rebooting my machine. 2017 MBP using a Belkin Thunderbolt 3 Express Dock HD to two HP monitors (1080). Dock is plugged into one of the left usb-C ports on the MBP. After rebooting, all three monitors (2 external 1 built-in) are working
_Ryan_S
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