Something's wrong with your system!
Althought GS3 seems more CPU hungry than GS 2.54,
in my XP3000+ CPU is about 3-4% (no fx).
This was about 1% of CPU in GS 2.54!
More or less otherwise GS3 works fine
and I really love it!
I saw something yesterday that scared me to death. A friend of mine sent me a screenshot of his task manager while GS3 was running... it creates like 3 or 4 different processes at the same time! I believe this may explain why your system is consuming so much processing power even when nothing is being played.