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Just found this, it looks promising.

http://www.serbianschool.com

Yeah I know. I didn't say only serbia, or say road signs in croatia are all latin because I would probably get attacked for that. For all I know there might be a tiny liitle town right on the border...

Excellent choice.

Three completely different languages. Serbian and Croatian are the most similar. Bosnian however has some Turkish influence I think. It'd be best to choose one and stick with it. learning all three...