Double_t func(Double_t *x, Double_t *par) {
   return par[0] + par[1]*x[0] + par[2]*x[1];
}

void myfit()
{

double x[9]={0,0,0,1,1,1,2,2,2};
double y[9]={0,1,2,0,1,2,0,1,2};
double z[9]={0,0,0,1,1,1,2,2,2};

TGraph2D *g2 = new TGraph2D(9,x,y,z);

TF2 *f = new TF2("f",func,0,2,0,2,3);
f->SetParameters(0,0,0);

g2->Fit(f);
TF2 *fresult = (TF2*)g2->FindObject("f");
fresult->Draw("surf1");
g2->Draw("same p0");

}