Some schools have specializations that they require you to name at some point & others actually do not - but either way when you write your application/essays you will need to know your post-MBA career goals & name the skill gaps you plan to fill via the MBA, so in that sense you actually have to indicate a focus even when you submit your application. The stronger & more clear your career goals & the skill gaps you need to fill, the stronger the argument you can make for why you need the MBA. That said, schools aren't going to hold you to those goals. Once you are in you can focus where you'd like.

And like I said some schools will require to name a concentration whereas others don't even have concentrations. Typically you wouldn't have to name one until at least your second semester though since you usually don't take any electives until then.
I wasn't totally sure what you meant when you asked about placement after 3 months so if this didn't answer that part of the question just let me know!