Curve fitting for more than three variables

I have performed a heat transfer experiment and I want to make a curve fitting for my data set.The study variable is Nusselt number (Nu) and it is a function of three variables Nu=f(Re,Ѳ,B).I have attached four charts according to four values of variable (Ѳ)and at each chart I change the variables (Re and perforation ratio(B)).So,how can I make a more accurate curve fitting for this data?