DiRT 3

Probably the hardest part of the game in terms of platinum difficulty. Every event in each of the seasons has a DC challenge attached to it that you will need to get the platinum medal on for this trophy. So do each of the discipline specific world tour events. There are 26 in total (The Gymkhana Academy in season 1 has 5). Take them 1 by 1 and practice until you get them. Trick steer assist definitely helps for challenges involving the relevant tricks so just keep it on at all time in the assist menu. Also remember you always have your flashbacks to fall back on. 1 mistake doesn't mean you should abandon your run. Again persistence is the key to this one, keep at it and the platinum medals will come.

Gymkhana Academy Tutorials

There are 5 of these located at the end of Season 1 in the Gymkhana Academy. They introduce you to all the major trick types of Gymkhana. For these, refer to the "Teacher's Pet" trophy for more information and videos of all the tutorials.
 

Drift Showcase Events

Drift Showcase events are a point to point stage where you go along and then back, drifting as much as you can to build up points. Really these events aren't too bad. You can just go fast and spin out most of the time and still get a 1-2 thousand point drift. You can't just go at a snail's pace the whole time though since it is timed, you don't have to reach the end or anything but it just takes your score either when you do finish or when the time runs out. There are 4 of these events, see the video below for a run of them all.

Gymkhana Attack Events

Gymkhana Attack is where around 6 different tricks are dotted about and you need to finish all of them in any order within the time. The key really is just getting a good route and then practising until you pull off all the tricks in one run. There are 3 of these events, see the video.

Gymkhana Sprint Events

Gymkhana sprint is very similar to attack although there are more objects/tricks and they have to be done in a set order. Again it's just practising until you nail all the tricks. Most time is lost at donuts and spins. If you ever have to go round a donut a second time or restart a spin then chances are you will just miss the medal requirement. There are 4 of these, see the video.

Smash Attack Events

Smash attacks are all about driving into cardboard robots and avoiding the cardboard buildings. It's all about knowing the route and practising. The route for the first couple is fairly straightforward but for the later ones it is a bit more open to interpretation but you also don't need to hit every robot for the platinum medal in those. There are 4 of these events, see the video below to get the routes for all of them.

Sprint Events

Sprint events are simple point to point speed runs where you need to pass through all the gates. If you miss a gate then flashback or restart because you almost certainly won't get gold. There isn't really much to say about these since the route is laid out for you, It's just about being as fast as you can.