We're actually in the middle of figuring out how skill activation rates are working. As a general consensus, we can't agree if the Cyrtanthus/Geranium/Justicia affection based skills stack additively, but are leaning on the possibility that the Moonvine/Tachibana 3rd-turn versions do.
I just got the triple-combo, so I'll be able to test that on Sunday/Monday, when I can evolve Justicia to obtain her Skill Rate ability.