I copied the following text from the article you linked above:
What three independent display configurations are supported?
Three independent displays are supported with most combinations of HDMI*, DVI, VGA, DisplayPort* (DP), Embedded DP (eDP), and Intel® Wireless Display (Intel® WiDi).
Are three display configurations supported using a USB display?
Three display configurations are not supported using a USB display.
I remember that the startech USB/VGA adapter has its own graphics card and says it will support up to 5 displays, presumably with 5 available USB ports.
The last quote must mean it won't support 2 USB externals. I've used the HDMI + USB for years on the old Sony I'm replacing, as well as on the HP Envy I just returned.
While USB/VGA adapter manufacturers may state maximum capabilities of their devices, our support states what we know to be working and the behavior we expect having completed validation with the supported configurations. USB/VGA adapters can vary in design and implementation; being unsupported doesn't mean it won't work, it means we don't guarantee which models will and won't since behavior is inconsistent.
Since this is a brand new HP laptop it should be covered under HP warranty, what did HP customer support have to say? Did you open a ticket yet?
Also, be sure you're using HP's latest released graphics driver for your specific laptop. OEM's often make optimizations in their drivers to fix issues and enhance features which would not be present in the generic Intel drivers.