you have to rename the file to c3 addon yes, but also the xml files of the plugins they need to change to C3 format or json format. however even if you do that, while the plugins will load in C3 they will only run in C2 runtime meaning most of the C3 features would be unavailable.
in order to have it fully compatible, you either convert all the plugins to work with c3runtime(basically redoing the plugin from scratch cause the SDK is different), or replace them with C3 alternatives, or events coding.