Have you looked at the Social Security Card. The last name (or number of last names) is printed on the 2nd line of the card. My guess is your clients do not really have 3 last names. They likely have two middle names and a last name. I know the parents are proud of their children and get upset when they don't see all of them, but this is sometimes necessary to get the returns processed. https://secure.ssa.gov/poms.nsf/lnx/0110205125#:~:text=Identifying%20first%2C%20middle%2C%20and%20la....