{ diff(x1, t) = (1*k13*k1*x1*(1 - x1) + (-1)*k13*(k2*x2 + k3*x3)/(x1 + k4)*x1)/k13,
  diff(x2, t) = (1*k13*k5*x2*(1 - x2) + (-1)*k13*k6*x1*x2/(x1 + k7))/k13,
  diff(x3, t) = (1*k13*k8*x1*x3/(k9 + x1) + (-1)*k13*(k10*x3 + k11*x1*x3/(x1 + k12)))/k13 }