The only way to do this would be to include C in your groupby (the groupby function can accept a list).
Give this a try:
df.groupby(['A','C'])['B'].sum()
One other thing to note, if you need to work with df after the aggregation you can also use the as_index=False
option to return a dataframe object. This one gave me problems when I was first working with Pandas. Example:
df.groupby(['A','C'], as_index=False)['B'].sum()