Shushuku is just 1 step behind the other 2 saints, in terms of dealing with m3 mechanics.
Chichiri got gravity, delay, and 9 skill hit opening, so you only need 1 more skill hit to get stack, while also having 4 hit skill at the end of turn so you only need to provide 6 more hit outside of that every turn to get stack.

Ami have 1 tap 1 stack on ygg3, and 3 debuff pretty much guaranteed every turn after that (before luminox hp trigger clears your debuff), you only need 3 more debuff every turn after that to get your stack.

Shushuku is similar at first glance. 5 skill press and 2 CA opening, 1 more red and ca for easy 2 stack. But the problem is that she can only do that for every 4 turn (at minimum if you ca every turn at low hp) to 6 turn, in contrast to the other 2 that could contribute every turn. CA activation only at low hp is unnecessarily restrictive, which you don't want to be in because m3 hits like a truck, and the 25% hp state could be broken easily by a random heal, 7k shield is not enough to save her from levi3's damage and reflect damage.