Synchronizing web, mobile and desktop apps using SQL database

I am currently researching and developing an application in Java for the desktop along with a Java applet to embed inside a webpage and I am using C# to develop an Android and IOS compatible app. (I am open to using other languages, these are just what I have started with and feel the most comfortable with at the moment).

I have it so that the user can use any of the 3 environments to enter a post which is then stored in a database.

How would I make the various apps sitting on each of the platforms pull the new post down from the database so that the users can see it on all three platforms straight after it has been added to the database.

I assume that the applications would need to be looking in a specific table which is owned by the admin who is logged in and doing the posting and then as soon as it sees a new post it could send a query to retrieve it. These came concept would need to be implemented to updating and deleting posts also.

I am new to this type of cross-environment development and wanted to ask for advice. Thank you for any help. I have tried to make this question as specific as I can considering I do not have a particularly specific question in the first place.

Any advice about how to go about this task will be greatly helpful.

Thanks.