If you use the Windows UPNP.DLL to do SSDP or use for example Intel's UPNP device spy, the Roku3 is not discovered. The Roku2 I previously had was OK.
There is a syntax error in the response from the Roku to the multicast message 239.255.255.250/1900 to discover it
The USN returned is uuid:31475333-444b-bece-0000-0503b00f0000:upnp:rootdevice and the proper syntax should be USN uuid:31475333-444b-bece-0000-0503b00f0000::upnp:rootdevice (note the double:: between UDN and service)