in the Tick event you are drawing only Tile Layer 1 & 2, you need to draw also Tile Layer 3!
Public Sub Tick (GS As X2GameStep)
mMario.Tick(GS) 'run first as mario updates the world position
TileMap.DrawScreen(Array("Tile Layer 3","Tile Layer 1", "Tile Layer 2"), GS.DrawingTasks)
End Sub
Note that i put Tile Layer 3 in the first position because it is a background and should be drawn first.
But if you ask me i would not draw the background using Tiled. it does not make much sense to use tiled to draw backgrounds.