For the mx-5 and 944 it's too long ago to really tell. The 944 doesn't have the most accurate engine sounds in FM3, that's all i can say.
The S2000 i can compare best, since i took it to the track this year and spent a full day pushing it to its limits (and over).
First of all, let me say this: I always found it very hard to compare a game to real life, no matter how much of a sim it is or not. What is really missing is all the forces... in real life i relate to feeling a lot, and don't really drive with my "head". So please take the following with a grain of salt...
In my opinion, the S2000 in FM3 is very similar to the S2000 IRL. The cockpit is done almost perfectly and the dials are spot on. Engine sound is also spot on, aswell as the powercurve/acceleration characteristics. I really feel like sitting in my car regarding those things.
Another thing that feels very smiliar to the S2000 IRL is the way how you lose traction in the rear sometimes. I'm not talking about a provoked powerslide, but loosing traction when going through a corner at the grip limit and reaching the point when it's slightly too much for the tyres.
http://www.youtube.com/watch?v ... 4VFj8&feature=related
Look at 4:20 in this video when i enter the corner after the straight. At 4:26 you can see how i push a little too much, and the tail quickly steps out (maybe hard to see, since it's just a slight angle). The way the tail steps out in the S2000 like that is something that feels exactly the same in FM3. It was the first car i bought in FM3 and i remember thinking "wow, they did a pretty good job simulating the handling of this car".
In short words:
Can't say for the miata and 944, but the S2000 feels a lot like its real life counterpart (as far as a game can feel like real life). Very good!
EDIT: Talking about the stock version of course.