About the Skill activation girls for those that missed it.

Girls with Turn 3 skill ups stack by adding the two together 1.2 + 1.2 = 2.4x chance
Girls with chance based on affection stack by multiplying 1.2 x 1.2 = 1.4x chance (probably 1.44 but it only shows 1.4)

So placing 3rd turn girls together is a good plan but affection based ones not as much. Having said that, I have 2 of each and star lilly (+15% atk on 3rd turn) in the same squad and they dominate. Most of the girls with these skills seem to also have high Defence so they take very little damage.