If I use a few 3rd party software instruments like Absynth, Alchemy and PLAY, only one core is being used, when all 3 tracks are playing back. This suddenly happened when I installed Logic 9. I just opened Logic 8 and it is now doing the same thing!!
Do you have all three tracks record-enabled or selected? That puts Logic into "live" mode, and all live tracks are played on a single core for minimum latency. If they are rec-enabled, try un-enabling them. If all three tracks are selected, try de-selecting them.