Re: There is a lot of traffic in Moscow during (a/the) rush hour
In BrE,#3 is the most common. #1 is used, but not as much as #3. #2, while not impossible, is not something I would recommend using. 'Rush hour' denotes period/situation at which there is a great deal of traffic travelling to or from work. The period can be shorter or considerably longer tha 60 minutes, so 'a rush hour' is unlikely. In fact, at the moment, I can't think of a context in which I could use it.
The BNC has only one citation:
... to try to miss some of the rush-hour traffic, though how you avoid a rush hour
which lasts from around seven in the morning to nine o'clock at night, ...
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