How to do sequential filtering for rankings

TopN 3 P/BV = 

VAR SelectedTop = SELECTEDVALUE('TOPN 3'[Top N 3])

RETURN
CALCULATE(
SWITCH(
    TRUE(),
    SelectedTop = 0, 
    [_Rank Switch P/BV],
    
    [_Rank Switch P/BV] <= SelectedTop,
    [_Rank Switch P/BV]
),

SelectedTop will never be zero. You would need to use the standard measure trick

VAR SelectedTop = 0+SELECTEDVALUE('TOPN 3'[Top N 3])

Adjust your other measures accordingly.  Note:  You may need to compute your TOPN dynamically because the TOP N 3 filter should only allow you to select whatever has been prefiltered in Top N 2.  (In your example Top N 3 should only allow you to select from 1 to 3 items, no?)