[01-yellow-ZX3]

3156 = single filament
3157 = dual filament

So in other words, a reverse light that only comes on and off would just require a 3156.

The front signals on a Focus with integrated park lights are dual filament. (park + turn signals).

Now I have no idea about rear sedan bulb requirements, but you don't need a 3157 for your reverse lights only.

3157LL is just Crappy Tire's "Long Life" bulb version.