Because is more common than as and since, both in writing and speaking. When we use because, we are focusing on the reason.
Because强调原因，更常用。
We often use as and since when we want to focus more on the result than the reason. As and since are more formal than because. We usually put a comma before since after the main clause:
As 和 since 关注结果胜过于原因。
Since 从句里一般会加逗号分隔。
当because , as ,since 在句首的时候，从句要用逗号分隔。
We use because, not as or since, in questions where the speaker proposes a reason:
Are you feeling unwell because you ate too much?
在疑问句中，只能用because
