ipythonのdisplay_formatterをnumpy.float64とか向けに設定してやれば
np.mean()がスカラー値を返す場合でも3桁固定表示に出来る
https://ipython.readthedocs.io/en/stable/api/generated/IPython.display.html#IPython.display.display

def float_formatter(target, pp, cycle):
____pp.text('{:.3f}'.format(target))

plain = get_ipython().display_formatter.formatters['text/plain']
plain.for_type(np.float64, float_formatter)
foo = np.array([1.234567, 2.3456789])
foo.mean() #=> 1.790
foo.mean(dtype='float32') #=> 1.790123