cause I thought they wouldn't do formals on the last night of a cruise
Even for the 3-nighter it isn't on the last night. You have casual on the first night, formal on the second night, and casual on the third night.
I am actually toying with the idea of not using the main dining room for dinner on my cruise. I still plan to tip the staff, but might do the casual dinner in the Windjammer each night. I haven't decided yet.