Limiting Processor Threads

Hi,

I'm on a 2011 MBP with 4 cores. I'm doing a fair bit of 3d-rendering in the background at the moment, and would like to do some admin stuff in Excel whilst the renders are being done.

Is there a software or system extension which would allow me to allocate the amount of processor cores/threads to different softwares?

For example, to allocate 3 cores to the 3d renders but keep one core separate to Excel?