it is a upside down of sentence. As was his customer= The dude who knocked three times was his customer. People use as to metion the situation in the past.

It's an example on Oxford Advanced Learner's Dictionary: http://www.oxfordlearnersdictionaries.com/definition/english/custom_1 In this example, 'as' is a conjunction which is used to make a 'comment clause': as: used to make a comment or to add information about what you have just said e.g. As you know, Julia is leaving soon. http://www.oxfordlearnersdictionaries.com/definition/english/as_3 Q: Why does the inversion happen? A: Inversion sometimes happens after 'as', 'than' and 'so' in a literary style. e.g. She was very religious, as were most of here friends. Michael Swan, Practical English Usage, Third edition, Page 280

In my opinion “as” is here a conjunction. What exactly “as” means I can’t tell you. I disagree with Darryl that there is not enough context to answer. The problem in my opinion is that most native speakers don’t have the sufficient grammatical knowledge to explain this - and neither do I. The sentence as a whole means something like "it was his habit to knock three times and that was what he did." Russell’s answer is wrong. “Custom” and customer” are two different words. "Custom"= A thing that one does habitually.

Hello Cherry. I will try to clarify this for you. In this case, ''as'' is a conjunction and it means ''because'', ''since''. So, the sentence would be ''Because it was his custom, he knocked three times.'' You can find here different uses of ''as'' http://dictionary.cambridge.org/dictionary/english/as I hope this helps :)

Uhhh... it is not entirely clear. I literally would answer your question as:" the person knocked on the door three times as a matter of habit." Sorry, there is really not enough context to give you an answer.