df1 = df1[['col1', 'col2']]
df2 = df2[['col1', 'col2', 'col3']]
merged_df = pd.merge(df1, df2, on='col1')
sum_values = merged_df.groupby(['col1', 'col2'])['col3'].sum().reset_index(name='sum')
result_df = pd.merge(df1, sum_values, on=['col1', 'col2'], how='left')