I am new to Java so go easy on me if this is a silly question.
I just got started with Java a few days ago and the first thing I did was build a helper class for MySQL. I am using
System.out.println("Memory Used: " + (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory()));
Setting them to null is unnecessary. The garbage collector knows the scope better than you.
You did well to close them all, but they should be done in individually wrapped try/catch blocks. If the first one throws an exception, you'll never close the others that follow.
You are worrying too much about something isn't likely to matter. It's not certain that the JVM will return the freed memory to the OS or that your tools will detect it. The important thing is that the garbage collector is doing its job properly.
This method won't kill memory on its on. You're more likely to find problems elsewhere in your code. Keep profiling with Visual VM to find them.