I Can confirm that turn 3 skills add together fully so 1.2 + 1.2 = 2.4x

I Can confirm that base skill ups add by multiplying 1.2 + 1.2 = 1.44x
Based on that i'd say 3 1.2x girls would be 1.73x (1.728 rounded up).

I can also say that having two 1.2 all the time girls and 2 1.2 at turn 3 girls in the same squad = close to 75% activation of skills for turns 1-2 and around 95% for turn 3.
Had a team setup like that for around 50 runs and that was the most common pattern.

I do not have Cymbidium to see how her 2x turn 1 skill effect others tho.