Are you asking about the 3 different lines for Switch friend codes when you go to add a friend code?
Some people have multiple friend codes so this gives them the option of adding multiple. Then, when they exchange friend codes with someone, they can tell them which number to use. If you only have 1, you can leave the other 2 blank or put the same friend code in all 3.
Let's go Gophers! Ski-U-Mah!
Sorry, I don't accept random FCRs.