I think SR3 was a huge step down from SR2 but I'd still recommend it over AC3, it isn't bad but it's by far the weakest game in the series(Well, actually AC1 might be worse but that one doesn't count anyway)
If you want an AC game, get AC2. It's great, if you like that one get Brotherhood and Revelations, personally I liked Revelations best of them because of the superior gameplay, the free running is way more fluid and you get many new awesome weapons. Many people say it's worse because of the story, but the story never made sense in the previous games anyway.
However, I think I'd recommend the Prince of Persia Collection over either of them, you get 5 awesome games for like 10 bucks.
Edit: nevermind, it seems that collection isn't available this year, what a shame.
If you want an AC game, get AC2. It's great, if you like that one get Brotherhood and Revelations, personally I liked Revelations best of them because of the superior gameplay, the free running is way more fluid and you get many new awesome weapons. Many people say it's worse because of the story, but the story never made sense in the previous games anyway.
However, I think I'd recommend the Prince of Persia Collection over either of them, you get 5 awesome games for like 10 bucks.
Edit: nevermind, it seems that collection isn't available this year, what a shame.