Opened an issue regarding this matter here: no suitable loudspeaker layout for higher order Ambisonic or arbitrary input signals · Issue #28 · steinbergmedia/vst3sdk · GitHub
With spatial audio technologies like Ambisonics or even WFS, the lack of an arbitrary discrete channel layout is a big problem for plug-in developers, making VST3 not suitable for that. Porting from VST2 is simply not possible at the moment.