@Tuoyo

Just one question. Are you driving with ASM turned off or on? If you turn it off, you'll have that effect, which blocks the wheels even at low speeds when you're turning, you'll have to compensate with a wider curve and higher apex exit speeds to counter that effect.
If you turn ASM on, the wheels will not lock, only at the most extreme speeds or side G decelerations. Plus, the wheels effect can also be felt with TCS turned off or on, you should check what options you have turned off or on.

It's rather simple... Forza 3's physics engine is just unrealistic and can't handle turning and breaking inputs at the same time.

I kid, I kid.... You should definitely be able to break while turning, while you don't get maximum brake efficiency unless you are traveling in a straight line it is still pretty useful. It's called trail braking and it helps to tuck your nose into a corner if done properly. All i can think of is maybe look at the aids, i hear forza will pretty much drive itself unless you take the assists off.

ah. Yes do NOT press all the way in or it will lock up. Think of pressing it in like a real brake. It takes alot of practice but you will get it. I have no problem at that speed braking in a corner on this game. Just remeber dont lock up. Try and perfect your braking before the turn in. Ease off on turn in. hit apex and accelerate out ( but gradually ). American cars tend to be the worst for power Oversteer. But then the best for drifting. :) 

You need to go into assist and turn ASM or ABS on.
Problem solved

I just saw this video demonstrating the physics of F3. I don't know if it has already been posted but if it hasn't, check it out.

http://www.youtube.com/watch?v=9iDTXYA_9nQ

Love the game played it until 2:30 last night. still i feel like a few things are missing. (i might be alone on this but i loved the" drivatar" from the first forza). Set up is fresh and newish. Anyone know if your doing the career races. (the weekday and weekend ones) If you can do more then when type at a time? You like FWD only on Wednesday and say mid engine rwd on friday. With out doing all 3 FWD races first? It would change it up a bit. Plus the weekend world champ series. So you could have say 3 different types going at once.

Cheers.  As I said I haven't played a racing sim since N64.  Back then there was just button presses.  You couldn't press a button down a bit or fully.  I have to try and get used to just tapping on the brakes button.  In fact I was going to see if there was a way to change config so that I could use A and B for accelerator and brakes but now I see why they went for the shoulder buttons.

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When you are playing a season is there a way to back out of a race tournament you don't want to play? I made the mistake of selecting an F Class Car and doing that thing were you do 3 heats against 1 car at a time and there are like 5 races. So you have to do about 15 of those head to heads. I have already done it with A and B classes and it was not very exciting to be honest. Doing it with an F Class Car is like as exciting as having my wisdom tooth taken out.

Had to waste 50k buying an X5 to make it slightly less tortorous. I have tried quitting after every race but waiting for it to load, quiting, waiting, loading over and over is killing me. Spent ages on it and had only managed to quit like 3 races. Managed to use the X5 to get me through one race. Still have about 11 to go.

 In sims you don't have peripheral vision like you would in the real world nor do you feel any G's either so 90 mph on a sim doesn't seem very fast.  Because of this you have to look far ahead from your car. I agree you should be able to brakes  some while turning but when it come to racing it's not ideal. Often using brakes when turning will cause you rear end to come around which it exactly what it's trying to do the whole time you are in the corner.

 When you hit the brakes the weight of the car goes forward to the front two tires. The harder you brake the more weight shift to the front which eventually causes the back wheel to lock up (low weight) and swing around (unless you have ABS).  Thus your front tires do most of the braking.

When you go around a corner the weight will shift to the tires opposite of the direction you are turning. The harder you turn right the more weight shift to the left two tires. In Nascar since they mostly turn left the right side tires wear faster than the left side since they are doing most of the turning.

Of course this is common sense but when you put both of them together you realize why it's a bad idea to brake and turn at the same time  even  with everyday driving especially in bad weather. That is it will shift most of the weight of the car to one tire. Two tires can stop faster than one. Also weight shifted to two tires can turn better than one. One tire doing most of the braking and turning could end up doing neither.

 In racing you want to do almost all of your braking before you enter the corner then use all of your tire grip to turn the corner then when you start to exit the corner use more traction toward acceleration.