In early seasons, when scenes called for filming outside the door of the trio's apartment, it is clearly seen that the walkway dead-ends at their apartment. However in the final season, when episodes centered on new neighbors next door, the walkway shows it continues on.
In the show's final seasons a grouped mailbox in the wall right outside of Mr. Furley's apartment. In earlier seasons, and even after airings of episodes that show the mailbox, there is no mailbox and only the wall is there.
The doorbell on the trio's apartment mostly was shown to be on the left, when facing the door, but occasionally for plot points it appeared on the right. Similarly as well the doorbell chime box inside the apartment originally was above the kitchen and later seasons it was over the front door.