How to reference one column by multiple tables columns primary keys

I have a database consist of 3 tables each table indicate one user like this:

1. 
   Admin table 'User 1'.
   
   
2. 
   Staff table 'User 2'.
   
   
3. 
   Student table 'user 3'.
   

I want to implement a common table between them called 'POST' WHERE each user can post content to the post table content column, however I need to Identify the type of posters ' the one who posted the post ' so I'm adding a column user_type to the table POST the column user_type should hold the ID of the USER ' Admin, Staff, Student '

I'm in a case where I want to be able to refer to this column ( user_type) in the POST table to multiple table columns Admin table, Staff table, Student table. So I Can identify the USER who posted the post.

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