I'm afraid I'd have to totally disagree with above, I think the traction and stability that comes with a 4 gear tranny completely outweighs any negative affects that a 4 gear tranny will have.
Having built and run an in-line motored 2wd car and realised how nervous and even unpredictable they can be on low grip tracks I'd rather go with an easy to drive 4 gear any day.
and any car with a 3 gear tranny just isn't as adjustable when it's in the air and won't have the traction you need ... I think that's why the CR2 has an idler gear TBH ...
4 gear all the way
and if yours is B4 based then just bolt in an X-6 tranny, the cases are only a tenner !!!