protobuf3 is in the Python Package Index.
To use pip to install protobuf3:
$ pip install protobuf3
To get a specific version of protobuf3:
$ pip install protobuf3==1.0.0
To upgrade using pip:
$ pip install --upgrade protobuf3
If you'd rather install directly from the source (i.e. to stay on the bleeding edge), check out the latest source from github and install the library from the resulting tree:
$ git clone [email protected]:Pr0Ger/protobuf3.git
$ cd protobuf3/
$ python setup.py install