find the exponent of a least square curve fitting

dear all,

i'm really a beginner, so i've some problems with the "least square curve fitting". i' ve two series of data for three "geometrical configuration" (indeed they are three curves):

for z=0.5:
y=[132 152 173]
x=[25000 34000 47000]

for z=1:
y=[128 148 176]
x=[ 25000 34000 47000]

for z=1.5:
y=[131 152 180]
x=[ 25000 34000 47000]

i need to find the exponent of the least square curve fitting that should be in the form of:

y=a*(x^b)*(z^c)

what is the script in order to find a,b,c?

i tried with basic fitting from tools but it doesn't solve my problem..

pleaseeee help me..

thanks