I just so happen to have all three.
This is my first review on cars so here goes...
Peugeot 908: This one is a complete beast. It is way over powered and way under weighted. Not that that's a bad thing if you can handle it, but it deffinitely gets tiring during the enduros, but if you have a DFGT, all you do is crank up the TCS a bit and you're good. Another issue is that you can't see very well out of the cockpit. It's the fastest out of the three and the cheapest.
Audi R10: The Audi is by far the slowest. And I mean by far. It's still good enough to beat the AI on all the races, even Bob can beat them, but it's not good for racing online. Stock, it understeers quite a bit, but all you have to do is buy some racing softs, and tweak the rear spring rate a bit and it will handle the best out of the three easily. Probably the best car to use out of the three for the enduros since it's not as tiring, and it's easy to drive and still quicker than the AI. It's also probably the best to drive from the cockpit view.
Mazda 787: This car is basically just a novelty item, just to listen to and drive around in. Faster than the Audi, but the handling is quite as good after some tweaks. I don't drive this one as much, so I'm not as much of an expert. In cockpit view is also good for this car.
After all that, I'd recommend the Peugeot, just because it's the cheapest. They're all great cars, and since they're all pretty much even, I'd go with the cheapest.