Does Cycles' GPU support Intel HD Graphics 3000? I'm currently planning on buying a Macbook Pro 13-inch from an acquaintance which uses Intel HD Graphics 3000.

While I was doing some research, I've read that there are people who say that Cycles' GPU only supports NVIDIA, but other's say that there are ways to make it work for Intel HD Graphics e.g. using Cycles AMD OpenCL or CUDA.
If they do work, how can I install them?
It will most certainly not work (ever). Refer to this page: https://wiki.blender.org/index.php/OpenCL
GPU on Apple products is a huge issue. Not even the extremely expensive mac pros come equipped with decent GPUs. So if you want to use CUDA etc... you're better off buying a laptop with a good nvidia GPU (You can run linux on it if you don't like windows (that's what I do)).