I've got a similar thread here No picture when connecting monitor via USB-C/Thunderbolt
Intel are looking into it, I'm hoping they'll find a fix soon as I really want a triple monitor setup.
Just out of interest, what monitors are you using ?
I don't think it's an issue with the monitors as my custom computer using a gtx960 worked with all 3, DP>HDMI
I'm using: Asus VS229HA 21.5 inch Widescreen Full HD VA LED Monitor (1920x1080, 5ms, HDMI, VGA, DVI-D)
It's either an issue with the USB Typc Adapter, or a driver/BIOS issue on the NUC which we need Intel to confirm.
I was able to get some information about this matter. Please check the following thread for more information: https://communities.intel.com/thread/107305?start=15&tstart=0