ETPC: Even at with all power diverted to engines and not missing a single gate, level three is still crushing me.
Ryan333: Its more important to shoot down targets then to go fast. Every turret you shoot gives +2 seconds on the clock. If you hit all 6 turrets on a platform, that's +12 seconds. There have been a few times when I've been able to finish a course with more time on the clock than I started with.
Excellent advice. Shoot as many targets as you possibly can (but not at the expense of missing a gate, since you're docked 15 seconds). Speed is really not all that crucial, as you're expected to be able to get through it with a Y-Wing, which is pretty slow. Indeed, if you find yourself going so fast that target fly by you, starve your engines of some more power (especially in the A-Wing). It's counterintuitive, but it works.
That said, the proving ground -is- tough. I've done all four several times (though today I keep PROVEN.PLT and TRAINED.PLT files so I can skip right to the tours and still have a pretty uniform), and my nerves are shot by the time I finish level 8; target fire gets pretty intense.
Also, as previously mentioned, the gates are just holograms, and can be shot through. They can also be flown through, and this counts as a pass for the gate in question, including the flap at the top (though -not- the bottom of those with a base---those are solid). It may also help to adjust your roll between platforms so that the next platform is always "down" when you approach.