Is it possible to modify oblivion via the construction set to get unlimited followers? Or, at the very least, 3 from each faction? (i.e. 3 dark brotherhood murderers/3 mages etc).

I can't seem to find any answers with regards to this. Anyone who has figured this out please post here, thanks

Unlimited followers aren't possible as Oblivion can't handle too many NPCs at once.

Maybe you can in a Bash patch?

You can set a limit in the bashed patch, but the limit exists regardless.

What's a bashed patch??

Also, is it possible, as I asked in my OP, to have 3 followers from each faction follow me using the construction set?

Does anyone here know??

Its a custom patch that you can create to make mods run better together,Unique tweaks,etc. You can get bash here -http://tesnexus.com/downloads/file.php?id=22368

You can read more about it on Tomlong's site ( i think) here-http://sites.google.com/site/oblivionpoinfo/index

Well, even if it's possible, it's not so recommended:
- When there're many actors(around 15~20 in a cell), you may experience performance hit(from noticable to severe).
- Also you can see brain-dead actor(actor without AI processing).
- When there're too many followers, they can block your path, especially you're in narrow passage.
- Followers are not so smart: when an actor hits another by accident, you can imagine the rest.

Much easier would be using a mod like Toaster's Companion Share.
As has been stated previously do be wary though.
Oblivion was designed as a single player campaign. Although modders have worked wonders with scripting they do have some very serious drawbacks.

I once tested how many NPCs I can handle before the game crashes and I got up to 100. That is with vanilla though. It is not possible to spawn even 30 in the same cell without major major lag when using mods(Rockmilk cave and FCOM, have fun with that ;p)