'As', 'because' and 'since' are conjunction that are used to give the reason of something. But it is a mistake to think that they are interchangeable. Let's see the difference between 'as', 'because' and 'since'.
'Because' is the most popular conjunction when we want to give the reason of something and when we want to focus on the reason. We use it both in writing and in speaking.
Becuase-clause can be placed at the beginning of the sentence and in the end. Also, we can use it on its own in informal speech and writing.
What's important: when we want to introduce the reason in a question, we use 'because', not 'as' or 'since'.
For example:
Police say Alshussein told troopers he didn't stop because he thought the officers wanted to race. (KOMO News)
Because I lost my phone, I could't call you yesterday.
Why did you get up so late? -- Because I watched TV till 3 a.m.
Are you going to work on Sunday because you haven't completed the project?
'As' is also used to give the reason but we use it mostly when we know the reason or the reason is not very important for us. That means that we focus on the result rather than the reason.
'As' and 'since' are interchangeable. These conjunctions are more formal than 'because'. As a rule, we place these conjunctions at the beginning of the sentence.
For example:
As John wasn't ready, I left the house alone.
Since this dress is very expensive, I couldn't buy it.
Some agents are embracing change as they navigate the health crisis. (WEAU)
Carol began to study well since she wanted to enter Yale.