Leadership Restriction states that: If a player chooses to place their second and/or third Vassal on another Leadership Space, they may not take advantage of any Vassal placement granted by that Leadership Action Space.
If you don't get the advantage of the second Vassal placement, how are you able to place a third Vassal? What's the point of placing the second Vassal in this case, since you don't get the benefit?
one of the leadership tiles has a space that allows you to place 2 extra vassals, which would be your 2nd and 3rd
the reason you might want to place without the advantage of the extra vassal placement is because maybe you want something else the space provides or maybe you want to block the space from an opponent.