how solve nonlinear equations with unknown constant?

Hello,

I have a nonlinear equations system, that it seems cant solve with equations nnlinear solver. can u help me?

my equation is : H = a - b*Q^c

i have 3 pair of (H,Q) that get from user for input to program and are unknown in first step and user should be determine...

and equations wold be:

H1 = a - b*Q1^c

H2 = a - b*Q2^c

H3 = a - b*Q3^c

now, and after set H1,H2,H3,Q1,Q2,Q3 by user, we should solve this program for find a,b,c variables. have u any suggestion?

note: I have a pack of H,Q. it means that I must calculate a,b,c and repeat this act for finding a1,b1,c1 by new H,Q pairs, and then a2,b2,c2 ... untile user determind at first step.

Best Regards.