Nah, the tempo and approach are different for each one.
Skin has nothing to do with that, only that their clothes becomes more vulgar, like of Terena had almost no clothes on EV26 but wore more but more lewd clothes on the beach in EV43.
All three were infected by the plant monster early in the story, but mostly Rietta and Arika who were injected by the monster, but it's more than that.
Terena fell fastest not because of the plant monster, but because of the bugs planted in her by the goblins, as they not only sucks her magic and knowledge each time she uses magic, they turn it into sexual desire and pleasure, reducing her from a magic teacher to a dumb whore.
Also, Kateve is more direct on having sex with her, as he already had done it before the 'Contract Period'
Rietta is zigzagged.
She may seem to have fallen faster than Arika, but she was brainwashed most of the 'Contract Period' into liking sex and the Pope-Or-Whatever, and had gotten out of it's influence by the end of EV44, but the changes to her body still remains, and (no need to watch the preview from the last stream to tell this) it likely remembers everything and so she will officially fall in the upcoming event.
The Pope did want to end it faster, but didn't want to break the brainwashing to early, so he was playing it safe until no signs of resistence remained, so he could go all out.
In general, Arika took the longest, as she the first girl, as well as the most stubborn one.
Arika avoided the goblin trap thanks to Kurt, but she wasn't spared from the Lord in the long run.
Lord Bonner kept making an 'offer-you-can't-refuse' several times to Arika, that only seems to reasonable and project her some of her dignity somehow, but Lord Bonner was actually trapping her into becoming his Sex Knight, banging her and training her into becoming his cum dumpster wife, which was slow but effective.
It is a longer route for Arika to fall to pleasure, unlike the two others who fell do to brainwashing and bugs, but that made her fall even harder on EV42.
