I would say fresh out of the shower after scrunching and diffusing I'm a 3a with the Kerapremium. Day two hair is a bit looser--rolling out of bed it's in the 2s. I wouldn't recommend this product for 3c girls who want to go straight, as I think it would take several treatments to lose that much curl. It seems to be a nice option for those who just want to defrizz and loosen up a bit. I'm pretty sure you could be extra conservative your first time around and only do one or two passes with the iron to make sure you don't lose too much curl, seeing as you could do another treatment on top of that if you needed to. I read reviews here and on Amazon about folks applying the treatment several times one on top of the next to get the desired effect.
Good luck with your decision!