Found the directories, done, worked well!
Thank you very much wonslung. In case anybody else wanted to know the exact directories - in my case, as I AM using http auth:
The 2.8 traffic data was in /dirwhereyouinstalledrtorrent2.8/plugins/trafic/stats/
And the 3.0 data was in /dirwhereyouinstalledrtorrent3.0/share/users/yourhttpauthuser/settings/trafic/
I just copied everything from the 2.8 dir to the 3.0 dir, preserving permissions, and all 2.8 past traffic data shown up beautifully in 3.0.