I thought - why not copy the “Curve EQ” plugin from my copy of Cubase 7 (Contents/VST3 folder), into the corresponding folder in Nuendo 5.5. So I did, and it works. Also the others seem to be working as well (eg “synthegine”).
I haven’t tried to see f they work if placed in the normal system VST3 folder, since I can’t think of any other apps that support VST3 anyway.
Not sure if this is “allowed”, but I will buy Nuendo 6 as soon as it’s released anyway, and I own both Cubase (two copies), and Nuendo 5.5, so “who’s the victim here”?