Why just 3 backups? Many small businesses keep at least 5 backups available to lessen the chances of having a bad backup that is not able to do a full restore of their data. The more backups you have available the better. What happens if a file is discovered to be missing because someone accidentally deleted it last week? How will you recover it?
With NTBACKUP, I create a directory on the disk for each backup I am going to maintain, and then schedule a backup for each day to go to a specific directory, overwriting what is already there. Each job is designated by the directory name of where it is stored. With a five day rotation, I have a directory named for each day of the week (Monday through Friday) and schedule a job to run each day. I am not sure how I would create a three day schedule unless it is only running 3 days of the week.