No. At this point you can only sort into a maximum of 2 categories. For the next release we are trying to change the app so you can sort into 3 categories. This will require switching to landscape mode and/or greatly reducing the size of the sorted images. We will try to balance the limitations of the iPad screen size with the need to add maximum functionality.
You are able to "sort" into only one category. This is a great way to introduce the activity to your student and make sure they understand the drag and drop motion required to succeed at the drill.