[ Eq(Derivative(x1, t), (1*k15*k1*x1*(1 - k2/k12*x1) + (-1)*k15*k3*x1/(k4*k12 + x1)*x2)/k15),
  Eq(Derivative(x2, t), (1*k15*(k5*x3/(k6*x3)*x2 + k8*x1 + k12*k13) + (-1)*k15*k7*x2)/k15),
  Eq(Derivative(x3, t), (1*k15*(k9/k12*x1*x2/(k10*k12 + x1) + k14) + (-1)*k15*k11*x3)/k15) ]