There are so many variables to consider when doing keratin treatments. I have been doing my own for almost 3 years. I am 3c as well, and some of them have gotten my hair bone straight esp the ones with formeldahyde). It mainly depends on the formula, the original condition of the hair (porosity ) and the amount of heat and passes.
I switched to cystine based "keratin" treaments because they are mild and pregnant women and children can use them. Plus, I can keep my curls.
I think QOD Max Organiq is excellent if want to keep your curls. Just do about 2 -3 passes on about 400 degrees and you should be fine.