The concept of an Anchor is that they are a period of time devoted to hitting PR’s, involve a decrease in overall barbell work, and an increase in assistance work. In my mind that equates to an increase in intensity and a decrease in volume. An Anchor is supposed to show how effective your training was in your Leader.
I would say for your Leader do 3X5 @ SSL and then in your Anchor do 3X5 @FSL. Generally I don’t think you do FSL sets for over 5 reps unless your running the Simplest Strength Template, running BBB @ FSL, or doing Widowmakers.
Regardless, by the time you hit your Anchor you the focus is on PR setting so 3x8 or 3x5 isn’t a really significant difference. If you are adhering to the core principles of 5/3/1 you will be fine.