OJS 2 plugins are not working in OJS 3 without adaptions. To simplify the update process for developers we (OJS-de.net) started a guide at the PKP wiki: https://pkp.sfu.ca/wiki/index.php?title=How_to_upgrade_an_OJS_2_plugin_to_OJS_3
Feedback and additions are very welcome. If you have access to the wiki, feel free to add stuff. If not add it here and I will add it to the wiki.