Get specific ArrayList item

mainList.get(list_index)

As many have already told you:

mainList.get(3);

Be sure to check the ArrayList Javadoc.

Also, be careful with the arrays indices: in Java, the first element is at index 0. So if you are trying to get the third element, your solution would be mainList.get(2);

Time to familiarize yourself with the ArrayList API and more:

ArrayList at Java 6 API Documentation

For your immediate question:

mainList.get(3);