What is the difference between 通常，经常，and 常常？
Generally, 经常 is used ahead of action, like 我经常加班

通常 is used to describe things happened frequently. like 事情通常是这样的

常常 can be used for both of the cases.

----- Edit -----

他通常说的都是对的.

我通常都是晚上开始工作.

both are okay to me too, you can see the differences. 

常常 can always take place of 经常 (anyway, I cannot recall any examples now that cannot replace it). 

通常来说， 常常 can also take place of 通常, but we just don't say 常常来说 :)