Simple. Use a different database for each application.
Why? Because they all have different table structures, and there's really no good reason at all to use a single database for all of them. What if you wanted to do something (e.g. upgrade) to one of your applications - you would impact all three of them while you did it.
What you can do is use the same database server (if not running it on the same server) to host the databases.
Security is not really a consideration here. One database or three, the security risks are going to be equal.
A side note suggestion - if you're going to be integrating those three applications and they're all hosted on the same server, you should still integrate them via their respective APIs rather than in the background in the databases etc. This means if/when your instances get big enough to warrant having their own servers, you don't have to change your code when they're all hosted in different places.