Because EMPHASIZES the reason. As and since suggest that the reason is OBVIOUS or less important.

I passed my exam with high marks because I spent a lot of time studying.

As the film had already started, we went up to the balcony and occupied some empty seats there.'
Since Peter had already eaten, I made do with a sandwich.

NOTES

1. Since can sound a little more formal than as, but the difference between as and since is more one of personal style.
2. As can mean BECAUSE or WHILE, so avoid confusion.

Ex: I was talking on the phone as I cleaned the house.

This could be interpreted in two ways,

I was talking on the phone BECAUSE I cleaned the house (reason)

or I was talking on the phone WHILE I cleaned the house. (At the same time as)

If there could be confusion, always use because.

Linkers: Expressing reasons with because, since, as.