Entering a corner
If you're a newcomer to automobile simulation racing, the most important Driver Aid you have at your disposal is the Best Line. Turn this aid on and a dotted line will appear on the pavement to show you where on the track your car should be positioned to best set up for maximum speed through the turns. Remember, entering a corner from the outside, cutting inside to the apex, then swinging back outside during the exit--using the curbs if you must--is the only way to drive.
Speed up the game
As is the case with most high-end racing simulations where exacting car control is crucial, nothing will ruin your day like an overburdened processor and the jumpy frame rate and/or slow gameplay it will cause. In GP3, depressing the O key while in the cockpit will reveal your current processor occupancy rate, a figure that must be kept under 100 percent. If you're running over that number, first try reducing the detail in your mirrors. If you're still high, drop the resolution to 800x600. And if that doesn't work, drop Trackside Objects to Low, then click the box beside "Reduced texture detail (Hardware 3D)." All of these modifications will make life easier for your processor without adversely affecting any of the game's most critical visual cues.
Properly braking in GP3 without locking the tires along the way is a difficult thing to do. A wise approach is to hit the brakes hard when coming to a turn, then back off as you (a) slow down and (b) hear the sounds of tire squeal and/or see puffs of tire smoke. Alternately, a quick succession of brake "blips" after the initial hard application should be enough to stop you while convincing the program that you're not locking the tires. And use those brake markers! If you see the 100 marker go flying by and you haven't yet started to brake, it may be too late.