And how would one go about doing this? Because all I could figure out from a google search is these two options: 1) Place one of the shells (exterior) of an apartment and use cheats to change the interior. But the outside always looks the same (more or less what the new pack will allow us to do). Or 2) Build a multi-level home, then use cheats to place a "hidden" room marker in blank spaces of the house that you don't want your Sim to live in.

The latter is not really "building an apartment" from scratch because no one is living the apartment but your Sim, there's no callbox feature, etc. It's basically just dressing up the outside of a big giant house to look like an apartment, but with none of the apartment features. If that's what you want, you can already do that in TS4 now, by building a big house that's styled as an apartment, then locking the rooms (apartment) you don't want your active Sim to visit. And hell, at least in TS4, you can have multiple households living on the same big, fake empty apartment lot.

So no, I don't think building an apartment could be truly done in TS3 either, at least not the way people want/the way it was in TS2.