is there any difference between "at the beginning , in the beginning , at the start"?
"At the start, ...", this is from the book, I am reading now. It sounded very rare to me so I was wondering what an occasion I can use this expression comparing "In the beginning, ..., at the beginning". Thanks.

Question

is there any difference between "at the beginning , in the beginning , at the  start"?
"At the start, ...", this is from the book, I am reading now. It sounded very rare to me so I was wondering what an occasion I can use this expression comparing "In the beginning, ..., at the beginning". Thanks.

Answer

'At the beginning/end' is about physical space and time e.g. at the end of the day (maybe 11pm), at the end of the street (maybe 100m down the street).
'In the beginning/end' is about a point in a story or series of events e.g. in the beginning God created the heavens and the earth (the beginning of the creation story in the Bible), in the end everybody in the movie died (the final event in the movie).
'At the start' is the only possible form, we can't say 'in the start'

Answer

All of those words are synonyms.  The most common phrase I hear is "in the beginning".  
When you say "at the start" it means immediately when something begins and all the time after that.