It depends on the context. AS could be used as an adverb, conjunction, and preposition. It more preferable.
Your context, we could use words like
Because is more common than as and since, both in writing and speaking. When we use because we are focusing on the reason.
We can use when to introduce a single completed event that takes place in the middle of a longer activity or event. In these cases, we usually use a continuous verb in the main clause to describe the background event.
We can use as to introduce two events happening at the same time. After as, we can use a simple or continuous form of the verb. The continuous form emphasises an action that interrupts or occurs during the progress of another action.