如果数组arr的长度可被3整除，则为

If your array arr has a length divisible by 3:

np.mean(arr.reshape(-1, 3), axis=1)

重塑为高维数组，然后对其中一个附加维执行某种形式的归约运算是numpy编程的主要内容.

Reshaping to a higher dimensional array and then performing some form of reduce operation on one of the additional dimensions is a staple of numpy programming.