as, since, because

[Brightsun]

I'd like to know the exact meaning of conjunction among as, since, because.
Can 'as' be changed to since or because?
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As it was getting dark, we soon turned back.
As you are sorry, I'll forgive you.
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Thank you so much in advance.

[Tdol]

In those examples, yes- they are all a way of giving the reason for something.