Have you already scanned in the assignment? You are not able to edit the assignment if you have scanned. You can clone the assignment and then add some questions - you are not able to pick and choose which questions you want to delete. If you change the number of questions from 15 to 12, GradeCam will remove the last three questions.
You can go in and make the questions you want to delete as "0" value so they are taken out of the equation of the overall number of points for the assignment.
Let me know if you have any other questions,