根据索引范围设置大熊猫列值

Question

假设我有以下pd数据帧

df = pd.DataFrame(np.random.randint(0, 10, size=(20, 3)), columns=list('ABC'))
>>> df
    A  B  C
0   7  4  7
1   4  8  7
2   8  8  3
3   4  8  0
4   9  4  3
5   5  6  1
6   9  1  6
7   6  0  5
8   4  3  9
9   1  0  5
10  6  2  1
11  2  2  7
12  7  9  1
13  3  5  5
14  0  9  1
15  5  5  3
16  0  1  3
17  6  1  1
18  1  4  7
19  4  8  0

我想创建一个新列D，它基于df的索引保存一些特定的值，即使该索引不是特别存在，而是将所有索引设置在特定范围内。到目前为止，我的尝试似乎无济于事

df.loc[0:5, "D"] = "one"
df.loc[5:10, "D"] = "two"
df.loc[10:, "D"] = "three"

基本上，我想得到一个看起来像这样的df

    A  B  C  D
0   7  4  7  one
1   4  8  7  one
2   8  8  3  one
3   4  8  0  one
4   9  4  3  one
5   5  6  1  two
6   9  1  6  two
7   6  0  5  two
8   4  3  9  two
9   1  0  5  two
10  6  2  1  two
11  2  2  7  three
12  7  9  1  three
13  3  5  5  three
14  0  9  1  three
15  5  5  3  three
16  0  1  3  three
17  6  1  1  three
18  1  4  7  three
19  4  8  0  three

如何实现？