There is a high chance that your elgg 2.x plugin will not support elgg 3.0. There has been lot of changes in the core due to which the plugin might not work.

Here's the list of changes: https://github.com/Elgg/Elgg/blob/3.0/docs/appendix/upgrade-notes/2.x-to-3.0.rst

If your plugin is developed by someone else, then you can contact the person to update your plugin and make it compatible with 3.0. Or, if you have developed the plugin then you need to make the required changes as mentioned in the link above.

Regarding the error_log, can you check phpinfo() once, just to be sure that the path for your php error_log file is correct.

Thanks Rohit. There are several Elgg core functions we used in our plugins that are now deprecated in Elgg 3.0.*. I am working on replacing those deprecated functions with newer ones.

About the error_log, now I can see my error_log by turning on the logging in mysite/admin/settings/advanced