non linear least square fitting

In terms of optical density dX at dose D and channel(X=R,G,B)X, these functions take the form dx =-log (aX+bXD)/(cX + D) aX, bX, and cX are the constants to be fitted. Rational functions have already the correct steady state limit case behavior for very high dose values D. Q1. now from graph how to find out these aX, bX, and cX are the constants to be fitted? Q@.below figure 1 represent my data and i want to know how theses aX, bX, and cX will be able to calculate from matlab curve fitting data or any other method

thanks.