The latest Python 3 formula links a program called python3-config. You can use it to find the headers like this:
python3-config --include
On my machine, this outputs:
-I/usr/local/Cellar/python3/3.3.4/Frameworks/Python.framework/Versions/3.3/include/python3.3m -I/usr/local/Cellar/python3/3.3.4/Frameworks/Python.framework/Versions/3.3/include/python3.3m
You may need to brew update && brew rm python3 && brew install python3 to enable this.
There is an equivalent program called python-config for Python 2.