Bill, I think that I misunderstood your initial question. I thought by "work well" you were referring to the mechanical compatibility with the rangefinder mount. However, I suspect that you are asking about whether the rangefinder base length on the R3A is sufficient to focus this lens. Is that your question?
I don't have an R3A, so I don't know from personal experience. But from what I've read, I think that the 90 mm lens will exceed the capabilities of this camera at f/2 - at least some of the time. However, since you already have the lens, why not try it once the R3A is delivered. The R3A is certainly capable of focusing a 90 at f/2.8. If you have half click f/stops on the 90/2, I would think that you'd also have a shot at f/2.4 - or whatever that half stop equates to.
Here's food for thought & some frame of reference. The R3A has an effective base length of 37.1 mm. The Leica M6/M7 .58 has an effective base length (EBL) of 40.16. Leica eliminated the 135 frame lines from the .58x magnification model but not the 90 frame lines. So, you might also post this same question on the photo.net Leica forum to see if you can find any users of the M6 .58 have experience with a 90/2 lens. I suspect that most of them are using a 90/2.8. The Leica literature claims that the EBL of the .58x viewfinder is sufficient to focus a 90 mm lens at f/2, but I suspect that it is just about at its limit; the R3A is close to the M6 .58, I think that it will fall short of the minimum base length requirement.
