The first episode will be "The Empty House", but I don't think it was a rat in it at all, (if it isn't a reference to Sebastian Moran himself). I've heard speaking of "The Sussex Vampire", it's something about someones last words being rat or something.

Wedding:

I think "The Sign Of Four" will be a little too obvious, I've read it and it was no wedding in it, only a proposal. Besides I realized it is a little to similar to "The Blind Banker". But I do think Mary will appear this season.

There is alot of "wedding's" in Sherlock Holmes, "Weddings" seems to mean "Trouble". For example: "A Case of Identity", "The Noble Bachelor", "Solitary Cyclist", "Charles Augustus Milverton" & " The Illustrious Client".

I've read none of those books, but I've googled a little around and my guess is that it might be based on "Charles Augustus Milverton". Which is about blackmailing.

Charles Augustus Milverton ruins alot of wedding by blackmailing and Sherlock takes the case. Sounds very likely to me. Pluss it contains one engaged Sherlock Holmes. I'm not even kidding!

Bow:

I don't think this will be the final season of Sherlock, because Moffatt said it would end as a cliff-hanger. It's actually a Sherlock Holmes Collection called "His Last Bow". One of those stories I think.

What I think:

Rat: The Empty House + some elements of The Sussex Vampire

Wedding: Loosely based on Charles Augustus Milverton

Bow: One of the "His Last Bow" stories.