{ diff(x1, t) = (1*k15*k1*x1*(1 - x1/k2) + (-1)*k15*k3*x1*x3/(k4 + x1))/k15,
  diff(x2, t) = (1*k15*k5*x1*x2/(k6 + x1) + (-1)*k15*k7*x2 + 1*k15*k8)/k15,
  diff(x3, t) = (1*k15*k9*x1*x2/(k10 + x1) + (-1)*k15*k11*x3 + 1*k15*k12)/k15 }