熊猫模块:

import pandas as pd
out_d = (pd.DataFrame(main_list).set_index([0,1])[2].astype(int).groupby(level=[0,1]).sum()).to_dict()
out_d

Out[1]:
{('user1@test.com', 'Administration'): 100,
 ('user2@test.com', 'Development'): 170,
 ('user2@test.com', 'Testing'): 30,
 ('user3@test.com', 'Administration'): 95,
 ('user3@test.com', 'Development'): 400,
 ('user3@test.com', 'Testing'): 200}

#for list
[[u[0],u[1],v] for u,v in out_d.items()]

Out[2]:
[['user1@test.com', 'Administration', 100],
 ['user2@test.com', 'Development', 170],
 ['user2@test.com', 'Testing', 30],
 ['user3@test.com', 'Administration', 95],
 ['user3@test.com', 'Development', 400],
 ['user3@test.com', 'Testing', 200]]