I think you can just do theta = pm.Uniform("theta", np.array([16.0, 0.4, 6.0]), np.array([18.0, 0.8, 10.2]), shape=3)
theta = pm.Uniform("theta", np.array([16.0, 0.4, 6.0]), np.array([18.0, 0.8, 10.2]), shape=3)