If you want
nhh = a * (n1^b) * (n2^c) * (n3^d)
consider taking logs
log(nhh) = log(a) + b*log(n1) + c*log(n2) + d*log(n3)
You can use any least squares feature in MATLAB, Statistics and Machine Learning Toolbox, or Optimization Toolbox. Pass in the log values as data. You'd get back estimates of log(a), b, c, and d.