I can’t think of a reason why the midi arp example should not work on vst3 if it’s only notes. Please provide more details about why and how it doesn’t work. I agree with you that juce should support as much of vst3 as possible, Notwithstanding the fact that vst3 cannot do everything with midi that vst2 can do. But still be forewarned, steinberg does not support or condone the use of either vst format for midi plugins! If juce arp is not working then get to the bottom of exactly why not. Most complaints about vst3 midi are related to non-note related problems.
Also does the arp example work when used inside the juce plugin host?
In my view steinberg does not condone or officially support vst midi plugin development. That means there is no Garauntee that all hosts will look for notes coming out of a vst3 plugin though that is likely to be ok if the midi buffer is passed by reference in and out of the plugin. Because it’s typically passed by reference then a plugin can manipulate the contents (notes only) and the changes will be seen by the host. But if the host copies the buffer there is no garauntee that the host will copy notes from output because steinberg doesn’t officially endorse or support that!!
Various hacks have been used by some hosts to handle cc and other types of events since vst3 abandoned them entirely. But not all hosts are doing that and it the same way so honestly I think if you need to process more then notes you should stick with vst2