%%cpp -d

#include "TF2.h"
#include "TH2.h"
#include "TMath.h"

double g2(double *x, double *par) {
   double r1 = double((x[0]-par[1])/par[2]);
   double r2 = double((x[1]-par[3])/par[4]);
   return par[0]*TMath::Exp(-0.5*(r1*r1+r2*r2));
}

%%cpp -d
double fun2(double *x, double *par) {
   double *p1 = &par[0];
   double *p2 = &par[5];
   double *p3 = &par[10];
   double result = g2(x,p1) + g2(x,p2) + g2(x,p3);
   return result;
}

const int npar = 15;
double f2params[npar] =
   {100,-3,3,-3,3,160,0,0.8,0,0.9,40,4,0.7,4,0.7};
TF2 *f2 = new TF2("f2",fun2,-10,10,-10,10, npar);
f2->SetParameters(f2params);

TH2F *h2 = new TH2F("h2","from f2",40,-10,10,40,-10,10);
int nentries = 100000;
h2->FillRandom("f2",nentries);

float ratio = 4*nentries/100000;
f2params[ 0] *= ratio;
f2params[ 5] *= ratio;
f2params[10] *= ratio;
f2->SetParameters(f2params);
h2->Fit("f2");

gROOT->GetListOfCanvases()->Draw()