Any EVO 1-3, 3.909 rear diff ratio centre (the entire guts inside the diff housing including crown and pinion) will be fine to put into your GSR diff housing if converting it to an EVO 3 drivetrain. You can still keep your existing GSR rear axles. Just don't get the EVO 1 GSR rear viscous LSD diff unless you get it complete with the cv cups and axles.
To save the hassle of swapping guts over and setting up tooth engagement and backlash professionally, you might be able to swap your GSR rear diff + $$$ for an EVO rear diff with an LSD in this forum as many guys with EVO's want to get rid of the 3.909 RDR and convert to GSR 3.545 RDR for lower rpm's at higher speeds on their street cars, or for longer legs in each gear on the track.
NOTE: The EVO owner will be losing his rear LSD in the swap. Although the LSD can take either the GSR or EVO crownwheel bolted onto it. It depends who wants the LSD.
EBay Seller by the name of EC_JDM based in Newcastle might be a good person to ask for an EVO rear diff.