要求保留小数点后三位，你能修改以下吗

当然可以，您只需要在最后输出Q矩阵的时候，将每个元素的小数位数设置为3即可。以下是修改后的代码：

import numpy as np

# 读入矩阵A
A = []
while True:
    try:
        row = input().strip().split()
        row = [float(x) for x in row]
        A.append(row)
    except:
        break
A = np.array(A)

# LU分解
n = A.shape[0]
L = np.zeros((n, n))
U = np.zeros((n, n))
for j in range(n):
    L[j, j] = 1
    for i in range(j, n):
        U[j, i] = A[j, i] - L[j, :j] @ U[:j, i]
    for i in range(j+1, n):
        L[i, j] = (A[i, j] - L[i, :j] @ U[:j, j]) / U[j, j]

# 计算Q矩阵
Q = L + U - np.eye(n)
Q = np.around(Q, decimals=3)

# 输出Q矩阵
for i in range(n):
    print(" ".join([format(x, '.3f') for x in Q[i, :]]))

输入样例：

2.0 1 5
4 1 12
-2 -4 5

输出样例：

2.000 1.000 5.000
2.000 -1.000 2.000
-1.000 3.000 4.000