What is the difference between 通常，经常，and 常常？
I hear them a lot in conversational Chinese, but I can't pick out the difference in meaning. The dictionaries I've checked don't seem to agree on their chosen English translations.
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 -----
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 :)
常常 simply means "often", and is not very formal. You wouldn't find it in legal documents and such.
经常 means "frequently" or "regularly". It's not just conveying the frequency of something, but also its regularity.
通常 also conveys frequency, but it's a bit more like "usually". Under such-and-such a condition, then something often or usually happens.
If I say 我常常在麦当劳吃饭, it just means that I "often" eat at McDonalds. But if it's 我通常在麦当劳吃饭, then when I have a meal, the "normal" or expected case is that I will be eating at McDonalds.
The difference is not really about the degree of frequency/infrequency.