Having played all three DiRT games, 3 is definitely the weakest. It's a pretty good game, of course, but it continues the series' lean into the Arcade-style of racing. It comes in full force in this game, where you're rewarded pretty heavily for driving recklessly, cutting corners, using other opponents as your personal turn cushions, etc. As usual, the damage system doesn't do much to mitigate this behavior, since you pretty much have to ram your car full speed at a concrete wall to end up crashing out of the race, and dynamic damage modeling that alters your handling of the vehicle for lesser crashes is near non-existent. The only time that ever came into play for me was during a single pickup truck race, where my front left tire blew out and made turning left extremely difficult.
The game is fun, but unfortunately extremely easy. I played on the second-to-hardest difficulty throughout and ended up getting in 1st in nearly every event on my first try. The pattern to winning is, do whatever it takes to ram your car to first place, then enjoy coasting for the rest of the race. The AI doesn't attempt to overtake you in any situation other than you screwing up a turn so bad you're practically facing the wrong direction. Not to say they should be driving just as recklessly as you're allowed to, but at the same time the AI is simply far, far too shy to overtake you. You truly have to screw up badly to lose 1st place once you have it.
One of DiRT 3's biggest new features is the addition of Gymkhana, a type of "exhibition" event where you drift like a maniac and crash into styrofoam. It breaks up the pace well, but the game's physics make this mode insufferable. The physics of the game were clearly tailored for classic Rally racing, where drifting isn't an absolute necessity at every turn, but in this mode you spend more time drifting than not. The wildly unpredictable controls and harsh penalties for even gently touching the walls add to the frustration. Thankfully you can skip right over these awful events in most cases during single player, but unfortunately you can't avoid them entirely.
Actual racing itself is greatly varied and a lot of fun. The game truly shines during the Rally events, where it's just you vs. some of the meanest, curviest, most dangerous tracks on earth. Rain and nighttime really do affect the gameplay, the former making the entire track a slip 'n slide ride, the latter limiting your view severely and forcing you to hone your reflexes. There is also snow, which I didn't enjoy as much due to the difficulty in knowing where the track ends and snow beings when it snows heavily, but it clearly affects gameplay and does provide some good variety. I haven't checked the hard numbers, but it seems like there are more event types than DiRT 2, which got a little boring with limited vehicle types. Here you have Rally vehicles, dune buggies, pick up trucks, and some truly frightening supercars. Unfortunately you don't get to drive a semi truck again like in DiRT 1 (which was just bizarre and a lot of fun), though. Events include Rally, trail blazer, head-to-head, and Gymkhana (ugh) among others. They're all basically variations of "drive the fastest", though, and the game's Tour mode really goes through practically every permutation of car+location+track+weather condition, so it does get kind of tiring toward the end.
One thing to mention is the complete absence of Codemasters' efforts in removing GFWL from this title. As of this day there's 1) no removal of the platform, and 2) no word from Codemasters about the effort other than "it's taking longer, sit tight". It means that all of the DLC is unavailable (of which there is a lot, and unfortunately the game really shoves it down your throat by mixing DLC cars and tracks among the car/track selection menu. Kinda tacky IMO) and in many cases, the game can't even start. I consider myself lucky in that I bought this game one month ago and played through it entirely with no GFWL issues on my Windows 8.1 system, but I know a lot of poeple aren't as lucky and many can't even log in. Shame on Codemasters for taking this long and really dragging their feet to fix their own game. I have no opinion on the mulitplayer since I didn't touch it.
Overall, DiRT 3 is a pretty fun romp into the world of rally racing once again (and an extremely good-looking one as well), though you should be mindful of the arcade-iest physics of the series (next to DiRT Showdown of course), the groan-inducing Gymkhana events, and the horrid GFWL situation.