How can I calculate the sum of 3 values from each number in a pandas dataframe including the first number?

I have a dataframe as below:

        Datetime     Data      Fn
0   18747.385417  11275.0       0
1   18747.388889   8872.0       1
2   18747.392361   7050.0       0
3   18747.395833   8240.0       1
4   18747.399306   5158.0       1
5   18747.402778   3926.0       0
6   18747.406250   4043.0       0
7   18747.409722   2752.0       1
8   18747.420139   3502.0       1
9   18747.423611   4026.0       1

I want to calculate the sum of 3 values from each number in Fn and put the value in Sum.

My expected result is this:

        Datetime     Data      Fn     Sum
0   18747.385417  11275.0       0       0
1   18747.388889   8872.0       1       0
2   18747.392361   7050.0       0       1
3   18747.395833   8240.0       1       2
4   18747.399306   5158.0       1       2 
5   18747.402778   3926.0       0       2
6   18747.406250   4043.0       0       1
7   18747.409722   2752.0       1       1
8   18747.420139   3502.0       1       2
9   18747.423611   4026.0       1       3

>Solution :

df['Sum'] = df.Fn.rolling(3).sum().fillna(0)

Output:

       Datetime     Data  Fn  Sum
0  18747.385417  11275.0   0  0.0
1  18747.388889   8872.0   1  0.0
2  18747.392361   7050.0   0  1.0
3  18747.395833   8240.0   1  2.0
4  18747.399306   5158.0   1  2.0
5  18747.402778   3926.0   0  2.0
6  18747.406250   4043.0   0  1.0
7  18747.409722   2752.0   1  1.0
8  18747.420139   3502.0   1  2.0
9  18747.423611   4026.0   1  3.0