Third-party means an addon made by a third-party, not Scirra.
So you should go directly to the author of a C2 third-party addon and ask them the question.
Some SDK is already available for C3 - https://www.scirra.com/forum/plugin-sdk_f196
Yes. What I meant was if the option is currently there for 3rd party users to add behaviours/plugins/effects?
I now see after the Beta was opened up that a C3addon file can be installed. I could not do so before.
How do we add additional effects?