Taxonomy Custom Column – ‘manage_{TAXONOMY}_custom_column’ filter only passing 2 arguments

You are using wrong number of arguments in add_filter, you specified to get 2 arguments and you are looking for third one:

Update your add_filter code to this:

add_filter( 'manage_post_tag_custom_column' , 'my_custom_column' , 10 , 3 );

The 3 in the end tells the filter to provide all the three arguments to your function.