Read through all the reviews of this movie. Big fan of the first two myself and just got back from watching this. Pretty surprised that nobody has articulated the real reasons why this movie doesn't work.
Firstly, if you didn't enjoy the first or second movie all of this probably won't be very relevant. I would have thought that most people watching the third would have likely seen and enjoyed the other films in the series.
So, why is this the weakest film in the series? Anybody's review who criticises it based on a unrealistic plot or implausible action is missing the point. This might be obvious but there are numerous reviews citing these as the reasons for the film being average/bad. It's also an important part of the formula that worked well in the first and second films.
The reason this third effort doesn't work as well and is not as enjoyable is due to a variety of factors - TONE and PACING probably being the key reasons and also areas where it departs from the previous instalments.
By tone i mean a few things. Is there ridiculous over the top action in this film? Yes, most definitely, but the problem is in how it is being presented. The other movies made you laugh in their almost comic book presentation and stunts. This film is genuinely trying to sell itself as a serious action movie. They completely took the humour out of all the potentially very humorous moments through the more 'arty' editing, over the top score dramatic score (soundtrack definitely plays a big part of the different tone in the 3rd movie) and generally lousy script.
This leads on to the pacing, which is far too slow at times. Way too much dialogue between Statham and the girl. Yes we know the character fairly well but the movies were always about the action and less the dialogue. Jason Statham is capable but the script can't really be saved here.
So, a dramatically more realistic Transporter movie but sucked dry of all the humorous and fun tone that made the previous instalments so much fun. As I said there are plenty of silly and deliberately humorous moments but the way it was constructed and presented meant I wasn't laughing anywhere near as much as I was in the other films. This in conclusion is the main failure of Transporter 3 - 5/10 just for being a new movie in an otherwise cool franchise.