how to do forward rolling sum in pandas?

Simple as:

df['B'] = df['A'].rolling(3).sum().shift(-3)

I believe need change ordering by iloc[::-1]:

df1 = (df.iloc[::-1]
        .groupby('A', sort=False)
        .rolling(7, on='B',min_periods=0).C
        .sum()
        .iloc[::-1])