Manual Multiple Regression without the curve fitting tool

I want to fit a data set for a custom equation.

f(X,Y,Z)=W=a0+a1*X+a2*Y+a3*Z
X=double(1*40)
Y=double(1*40)
Z=double(1*40)
W=double(120*40)

I need to fit these data i=120 times for each W(i,40)and determine the set of constants(120 sets) and the error(in 95% confidence interval) . I cannot do this in curve fitting tool as it accepts only 2 data sets. Any help will be appreciated.