groupby fillna ffill

1df['three'] = df.groupby(['one','two'], sort=False)['three']
2                .apply(lambda x: x.ffill().bfill())
3print (df)
4   one  two  three
50    1    1   10.0
61    1    1   10.0
72    1    1   10.0
83    1    2   20.0
94    1    2   20.0
105    1    2   20.0
116    1    3    NaN
127    1    3    NaN

1print (df)
2   one  two  three
30    1    1   10.0
41    1    1   40.0
52    1    1    NaN
63    1    2    NaN
74    1    2   20.0
85    1    2    NaN
96    1    3    NaN
107    1    3    NaN
11
12df['three'] = df.groupby(['one','two'], sort=False)['three']
13                .apply(lambda x: x.fillna(x.mean()))
14print (df)
15   one  two  three
160    1    1   10.0
171    1    1   40.0
182    1    1   25.0
193    1    2   20.0
204    1    2   20.0
215    1    2   20.0
226    1    3    NaN
237    1    3    NaN