Are you sure that was a comment on Thief 3 and not Thief 1 and 2. I know the first two had multi-core problems but I didn't think the 3rd one did.
I don't have a link to the tool but there are two other ways you can fix the problem. After starting the game, Alt-Tab out of it (assuming the game doesn't just crash by doing that). Then open the task manager, right click on the thief process and select the "Select Affinity" option and uncheck all but one of the CPUs. Unfortunately you'll have to repeat that every time you start the game.
You should also be able to disable the hyper-threading function of your CPU in your bios although I don't know if you can disable entire cores. But thats a route to try at least to see if it makes the game work.