If you have the source code and use JUCE then yes, you can convert the plugin to VST3. But if you don't use a framework then you need to port the code manually.

Notionally yes but the idea of keeping the DSP and the container logic separate is relatively new and lots of projects didn't do it.

(Actually the idea is very old and how most LADSPA plugins were made but some time in the mid oughts everybody forgot about it.)

If you're using JUCE and not using any of the VST2 features removed in VST3.