Not knowing how your data table is structured makes it pretty hard to provide an answer to your question. I assume that you will need to create a new column and then to apply a formula to the column that sums up the 3 Likert Scale columns.
I suggest that you read the Discovering JMP document that can be found under the Help pull down menu. I should give you some assistance.
Also, if you could supply a sample of your data table, or a more detailed description of your data table, that would also be helpful in evaluating the solution to your question.
Jim