I used a ds3 on gt6 and platinum the game. I'm not a very good racer overall. Well at least on the gt series.
lol, that's no help at all. neither is telling someone to get a wheel when they ask for DS3 help. A couple of things could be used to help you.
First of all, if you are using racing tires... stop. On street cars it makes it near impossible to keep control because it's hard to use the finesse needed to deal with that much grip. I know it sounds stupid but try sport soft tires and have your mind blown.
Also, bring your steering sensitivity down to -2. most of the time when you start to oversteer it only takes a small amount of correction to fix the problem and that's hard to do with a DS3. with the lower sensitivity you can just flick the stick and your tires won't turn all the way. It's a small adjustment but makes a difference.
I would recommend uning the right stick for throttle/brake and having a second brake button like R2 or something. it's more wasy to make small throttle adjustments while using the stick but you can still manipulate gas and brake together using the top button.
Try shifting into the next gear for a split second when exiting a turn. When you go from full braking to gas in a corner there is a lot of weight moving around as well as torque trying to move weight around and it's more easy to break the rear tires loose. If you are coming into a 3rd gear corner then shift down to 3rd as you are braking but shift back to 4th when you use the throttle to exit. It takes less than a second for the car to get settled, then you shift back to 3rd for your acceleration.
Next is to slow down. it's better to go too slow than too fast. too slow keeps you on the track. you can't drive a track at 100% until you KNOW that track and your car.
Hope that helps, the sensitivity and gear changes will make the biggest difference for you i think. unless you are always on race tires, in that case the tire change to sports will be the biggest difference.