If you are using a metal sheathed wire...(BX or MC Light), there is no need nor is it permissible to run it through a conduit.You may find a 12/3 with ground but you haven't stated what you are wiring so I can't really answer this specifically.

Mr Handy

I assume you're talking about flexible metal conduit and then pulling wires through it. BX has wires already permanently installed inside it.

Just my opinion, but it's always easier to run the wires through a larger conduit. So if you can fit the larger conduit through the wall, and the price isn't much more (it's usually not), I'd go with the bigger one.

The flexible isn't as smooth inside as rigid EMT, so pulling the wires is a bit harder. So the larger size would be easier.

Either size will work as far as meeting code. (Frankly, the new code allows more wires in a conduit or box than I could ever make fit reasonably, so I don't worry about that much any more.)